I've been thinking about this quite a bit, and for me it comes down to a violation of genre expectations. In a superhero movie, the heroes save the day. It might not be a complete victory, and there may have been some heartbreaking sacrifice to get there, but in the end it still feels like a win. It reminds me of the debate regarding whether a romance novel needs to end with a "happily ever after". You can write a wonderful story about a romantic relationship, but if it doesn't have a HOA (or at least a "happily for now") ending, it's not a romance novel. Similarly, by not giving us a victory, and by leaving the surviving characters in utter despair, this ending violates the norms of the superhero genre. And I suppose that's not inherently wrong, but it leaves me feeling unsatisfied.