i'm not a comic book movie fanboy by any stretch, but the 8-10 good to great movies (out of like 25 total) that end the story with infinity wars and endgame is fantastic storytelling through film and anyone who says otherwise is a fucking hater and or pretend movie-snob/dork.
i understand what you're saying, but in actuality, it's the opposite. the reason there are so many headliners is because they all headlined their own origin story movies, and then the movies started overlapping casts with each other. so yes, is the answer to "roles nobody else could've played". when cumberbatch plays dr strange in a decent movie with a so-so cast, you understand his story and role, and then when his story overlaps with other stories, all the main stars are part of it.
those last two movies bring a lot of shit back from literally 12 years of movies, so pretty much everybody in any of those movies appears in the final movie. none of them were cameos, they were all reprising roles they played in previous movies, if that makes sense.
it's a lot of time, but like i said, the ~10 movie journey is fucking worth it. pretty remarkable cinematic achievement.