I honestly think it moreso has to do with Whedon wanting a happier, wrapped-up ending, rather than something like TWS that was more bittersweet and open-ended. That's something people really don't bring up in regards to AOU I've noticed, that Whedon seems to have a very specific idea of what a superhero movie should be - played completely straight and operating on tried-and-true tropes, which is what made the first movie so sweet. But his desire to make a darker movie while upholding those principles sort of butted heads with each other.
The movie played out pretty close to the first one structurally & content-wise, but through a slightly dour filter in tone & look, so the (mostly) peppy nature of the scenes at the end felt unearned despite the attempt to sweep any substantial consequences under the rug. Sure, there's a roster shakeup, but it doesn't feel like that comes at the cost of any major issues from the original team members, with the exception of Banner. The mood of the Big Three's final scene is very chill, including for Thor, who casually talks about heading out to stop an impending apocalypse he's had nightmare visions of.