Great literature. Our contemporary myths. Timeless art. I wonder what the people who worked on funny pages back in the 30s and 40s would think if they knew their work would form the basis of the most successful entertainment properties not just of the era but in history almost a century hence.

Not only that, that adults the world over would regard their stories the way our ancestors regarded works of art from The Iliad to Moby Dick.

Crucially, the operative term above is 'adults'. For most of history, while adults were absorbing the social themes in Heroditus or Plato right through to Huxley, Orwell or Harper Lee, kids were playing with dolls and reading comic books.

Today, adults love characters who dress up in silly costumes and battle intergalactic wars. As Bill Maher intimated (angering many), it's as if people have lost the ability to put childish things behind them when they grow up.

It's also interesting how much the world as a whole responds to comic book superhero movies where there isn't a corresponding surge in interest in comic books and graphic novels themselves. They've of course enjoyed a much higher profile not just since the advent of the Marvel Cinematic Universe but the time when anime and manga became popular around the end of the VHS era.

Marvel in particular (it's something Warner Bros/DC haven't quite managed) weave some alchemy that make their movies hits, even when I'm sure 19 out of 20 people you approached on the street wouldn't have had any idea who Black Panther or Captain Marvel were before there were movies about them – even Iron Man back before 2008.

And that's irrespective of how many people read comic books. Even despite the economics of the situation, it's movies that are now the apex property, any comic book or toy sales just ancillary revenue while the movies enjoy all the profile and cachet.

All of which makes me very bemused that these movies are so popular. As I write these words, Marvel is about to re-release Avengers: Endgame, and in doing so it could very well become the highest grossing movie in history – it surpassed Titanic weeks back and is now only $30m short of Avatar's triumphant $2.787bn haul.

After all, they're essentially still stories for kids, all bright colours, flashing lights and noise with no real nuance, stories about good and evil as plain as there's ever been.

Maybe we're all so overworked and exhausted by the demands of the modern world (steady employment is less certain than ever, political unrest is getting exhausting, etc) we want to check our brains at the door and be fed bread and circuses instead of think and be challenged more than our parents ever did.

Or maybe the alchemy Marvel wields just makes these movies as fun as hell. I'm still not the biggest fan of any of this stuff, but you can't deny the impressive universe-building that's gone on across more than 20 movies. It means that at the very least this isn't the usual superhero origin story we can all recite in our sleep.

Like Return of the Jedi, we meet the good guys in defeat and disarray. Thanos (Josh Brolin) has snapped his fingers and wiped out half the life-forms in the entire universe, retiring to the cabin on his farm planet.

Tony (Robert Downey Jr) is saved from drifting in space in the crippled Guardians ship by Captain Marvel (Brie Larson), but when he returns to Earth he's angry and defeated, arguments breaking out that see him leaving to live his own life with Pepper.

Even when Carol shows them where Thanos is holed up and they go and kill him like they should have the first time, it doesn't bring much closure to anyone. The last act of the Mad Titan was to use the stones' power to destroy them, so what he did can never be undone. Thor beheading him is a hollow victory.

After everyone's fallen apart, the title card reading 'Five Years Later' prompts the same frisson of excitement among the cinema that the '19 years later' one did at the end of the last Harry Potter movie.

Some of the surviving Avengers are trying to hold what remains of the world together and some have scattered to the wind. Natasha (Scarlett Johansson) stays at her post in Avengers headquarters because it's all she's ever known. Steve (Chris Evans) is as depressed as anyone, but his essential goodness means he's trying to encourage everyone to heal and move on, even sitting in on support groups telling people it'll all take one step at a time.

Clint (Jeremy Renner) has taken the persona of Ronin, stalking through the streets of Tokyo killing gangsters, Thor (Chris Hemsworth) has become a slob, a shut-in and a drunk in a seaside village with nothing left to rule over, crushed by his failure to defeat Thanos when he should have.

But other members of the team are thriving. Bruce (Mark Ruffalo) has accepted his alter ego and become Hulk full time, albeit a smart and well-spoken one (apparently it's a beloved motif from the comics but it comes off as slightly silly on screen). Tony has done better yet, having a little girl with Pepper and living the life of Riley in a lakeside house.

But the inciting incident is when Scott (Paul Rudd) returns from the quantum realm we last saw him trapped in from Ant-Man and the Wasp. The real hero of the entire MCU is actually a rat, unwittingly activating the quantum tunnel machinery in the back of Hank Pym's van, now rusting away in a San Francisco storage facility and bringing him back.

Scott staggers home through a city ruined by decay, with no idea of everything that's gone on and finding his cute daughter now a teenager. But when he finds his footing he goes to Avengers headquarters with a bold idea – after spending only five hours in the quantum realm where it was five years outside, maybe they can manipulate time to collect the stones before Thanos gets them, using them to snap him out of existence.

Step one is to convince Tony to take part, but while he's welcoming to his old comrades he wants no part in risking everything to try and save half the universe – he's got his family and home and that's all he wants. But Tony can't help himself, and he uses Hank, Janet and Hope's calculations from the quantum realm to crack the problem of time travel.

The remaining Avengers pinpoint periods and places where they know the Infinity Stones could be found and scatter across recent history in teams to get them, from the planet where we first meet Peter Quill (Chris Pratt), the one where you have to kill someone you love to get the stone for the Red Skull guy out of Captain America: The First Avenger, and the 2012 battle of New York where the Avengers first fought Thanos' forces against Loki (Tom Hiddleston).

The plot of each team finding the stones – often in the midst of pitched fights and trying to either avoid or beat earlier versions of themselves – is very well constructed by writers Christopher Markus and Steven McFeely.

It makes for some great fun sequences, like where Cap fights his earlier self (who believes the temporal interloper is actually Loki in disguise) or when Tony – having gone back to the early 70s to find more of the material that facilitates time travel from a young Hank Pym's lab – runs into his father Howard (John Slattery) when they're the same age.

But eventually it all has to go pear shaped, early Thanos tipped off when it turns out the earlier, still-evil Nebula (Karen Gillan) is mentally connected to her later self, knows the whole plan and leads Thanos right to Earth not to finish but surpass what he started with the snap.

So many faces from across the MCU show up it makes the Stan Lee cameo just one of many, with Robert Redford, Natalie Portman, Hayley Atwell, Marisa Tomei, William Hurt, Tilda Swinton, Taika Waititi and many more all making appearances.

All of which makes the final battle pretty ambitious. The cast is crowded enough even without everyone who died in the snap, and when they all come back for the final showdown around the ruins of the Avengers headquarters, there are so many hero (literally and technically) shots of Dr Strange (Benedict Cumberbatch), Spider Man (Tom Holland), Valkyrie (Tessa Thompson), the returning Guardians of the Galaxy, T'Challa (Chadwick Boseman) and his forces and everyone else joining the battle there's barely any time for them to actually fight.

After two decades of giant CGI armies meeting on battlefields thanks to the Lord of the Rings effect, it's hard to make a fight scene bigger than any we've seen before, but directors Joe and Anthony Russo do their level best.

In a movie this big (3 hours) and expansive, there are bound to be both plot holes and slightly silly turns in the story. Early on, Captain Marvel tells the Avengers she might not be around for the fight against Thanos because she has other stuff to do elsewhere in the universe. When she arrives in the middle of the climactic battle (just in the nick of time, natch), I wondered what could be more important than stopping the guy who wants to destroy every living creature in the universe.

And even though Hawkeye becoming Ronin in response to losing his family in the snap was apparently a popular development in the comics, it's merely nodded to in a single scene here and then forgotten, Clint's dark, angry new assassin's personality jettisoned just as quickly when he comes back to join the time travelling effort.

But there are so many callbacks, cross-pollinations and payoffs across the MCU you can consider the entire decade to be one big budget event series with episodes lasting 90-120 minutes that just happens to have played on cinema screens rather than Netflix.

Not long after the movie came out a short clip resurfaced from Avengers: Age of Ultron where they're all sitting around drinking beer trying to pick Mjölnir up off a table. After a series of fruitless attempts by the team Steve manages to jostle it the tiniest bit, the frame focusing on Thor in the background and his self-satisfied smile disappearing.

As well as being just so much movie crammed into three hours (the ultimate test is how aware you are of the time passing, and you breeze through Avengers: Endgame effortlessly), it's also – thankfully – the conclusion to a lot of other stuff rather than an introduction to another new face taking you on the Marvel roller coaster we're all so familiar with.

It's the end of something that's never been done in cinemas to this scale before, and however much we complain that movie screens are only home to superhero movie for kids, that's kind of historic.

But if you think it's the end of Marvel movies after the $2.75bn haul, I've got a genuine infinity stone I'd like to sell you.