When you try to make a movie where a big event happens and the heroes lose, the impact is completely lost if the audience doesn't believe there are any lasting repercussions and that the big event is just going to be undone in the next movie.
Except the last 5 or 6 Marvel films (with the exception of Captain Marvel, which was a prequel) have ALL been about the repercussions of past events. Ragnarok covered the aftermath of The Dark World and Age of Ultron, including the death of Odin as a direct result of Loki's actions. Black Panther was all about the aftermath of Civil War, as was Ant-Man and the Wasp. Infinity War and Civil War both called back to events that have occurred since literally the start of the MCU, with the latter basically being ALL ABOUT the actions of the heroes having consequences. Endgame is the first time they actually reversed a major event and it came at the loss of three original Avengers, one of the main Guardians and immense changes to most of the other cast.
I mean, you can point to The Dark Knight in the very same genre for a film that is willing to actually have stakes when the main love interest and one of the main heroes suffers and the Joker arguably wins in the end in way that can't simply be erased away.
Except... the next movie basically ignored everything in The Dark Knight except the last five minutes. None of Joker's actions are shown as having a lasting impact. Now a lot of that can be blamed on Heath Ledger's death, but that doesn't change the fact that very few consequences are seen.
I can imagine audiences at the end of Empire Strikes Back were wondering what would happen next, because at least they weren't fore-advertised that Han Solo has a new movie coming soon.
Because Empire Strikes Back wasn't made in the days of the Internet. Secrecy regarding future plans is borderline pointless nowadays because there are media reports of almost every major occurrence on film sets. It would have been obvious FAR in advance that Han wasn't dead if Return of the Jedi was released in 2013, rather than 1983.
I mean Obi Wan dying or Gandalf has more impact that any death I can think of in a Marvel film, outside a villain like Killmonger, and both of them come back as well, but are at least changed by the process in some way.
Obi-Wan isn't really changed at all—he's literally the same character, just in the form of a spirit guide. Gandalf is pretty much EXACTLY what you're complaining about in the MCU—a major event being effectively retconned in the next book/movie. None of the changes to Gandalf required his death in the interlude.
That's just me trying to keep to "blockbusters", and I don't like being nice to certain films because of their genres either. Seven Samurai, The Godfather, Saving Private Ryan, and countless others are set up to tell stories with pathos and stakes, not to coddle their characters to make more money with more films later on.
The Godfather had two sequels. Saving Private Ryan was a heroic sacrifice movie. Also, funny how your examples are literally some of the best films EVER MADE, compared to the vast, vast majority of films which follow predictable patterns.
Marvel's film franchise is compared to television because of their quantity, but compare it to something like The Wire and you can see what I mean about coddling characters.
Because a story about inner city drug dealers is fundamentally different from one about Superheroes. The MCU isn't and shouldn't be like the Wire, anymore than it should be like a Rom-Com. They're completely different genres. Killing characters because you can or making stories dark for their own sake isn't automatically good storytelling. It's more often a lazy cop-out for shock value that only occasionally works. Iron Man dying after 20+ films of buildup is far more satisfying than killing him in the first Avenger's film just to create a false sense of "stakes".
If Evans and RDJ wanted to make more films, they would not have the ending they had in Endgame. I would bet on that.
Now you're just being disingenuous. "They killed two of their main characters, rendering the entire argument about no stakes facile... better pretend that it was because the ACTORS wanted out". Go look at the Box Office take for Endgame, do you really think there is ANY amount of money those two could require for future appearances Marvel couldn't afford if they had wanted to keep the characters, even if just for cameo purposes?
I like them, but they haven't ever made a masterpiece film in my mind and I don't think they have ever competed with the big Blockbusters of film canon.
Infinity War and Endgame had probably the largest ensemble cast ever assembled in meaningful roles and not only paid off 20 films of prior buildup, but introduced one of the most distinct cinema villains in a very long time and did it all while making excellent movies. There are a lot of so-called "masterpieces" which can't claim nearly so much.
Not really a controversial opinion in the film community
The film community gave an Oscar to Crash and nominated "The Curious Case of Benjamen Button" over "The Dark Knight", despite the latter having a performance so compelling the academy literally set aside their own rules about posthumous nominations for Heath Ledger. "The film community" saying something is irrelevant because the film community are so obsessed with elevating the importance of the medium that quality doesn't matter. They prefer "important" movies about topical issues and dark, dramatic acting over actual quality. This wasn't always the case. Go back 50 years and a massive set-piece movie would have been considered a shoe-in for an Oscar nomination. Star Wars got a Best Picture nomination, despite the fact it's literally just a standard hero's journey and NO ONE denies that fact. Likewise with Raider's of the Lost Ark. Blockbuster's haven't gotten worse, Avengers Endgame isn't somehow deeply inferior to so-called masterpieces of film canon. The film community have just decided that popular=unworthy and so flaws that would never get mentioned in a classic film like Star Wars become an unforgivable sin in an Avenger's film.