Would a montage of widespread death make the movie too bleak? No, because this montage would be the setup, and the payoff would come at the film's end, when we see all these people in different countries and different planets miraculously return. Incredibly, the film we got never actually shows us the return of all the people the Avengers saved, other than by showing those the heroes need personally.
Walt Disney Pictures
Instead of reunions across the universe, we close on just the heroes happily reunited with their own families. And some of these aren't reunions at all. Ant-Man and The Wasp cuddle after being separated for mere days. Ant-Man and his daughter cuddle though neither was snapped, and they already reunited at the start of the movie. Peter Parker and Ned embrace like they've been apart for years, though both were snapped and so neither had a chance to miss anyone. We're only allowed to care about the heroes and their friends.
I'm not saying every movie has to be filled with fawning, grateful randos. I don't even really like those scenes I mentioned from Thor, Iron Man 2, and The Avengers, and my favorite climax in any MCU movie is Civil War, where the conflict is totally character-based and no one's trying to save anyone. But this film was set up to be about the loss and resurrection of trillions, and yet from watching the finale, it might as well have been about the heroes saving just their own loved ones. We don't get to ever see the ordinary people rejoice. We're left watching Avengers literally admire their own asses.