The heroes are seeing the "alternate selves" much like a parent sees their children. Yes they will have some form of nonexistence / death / alternate life like waking up from a dream and forgetting the dream. Yet ...
This sentiment well it is a very comic book type ending.
After all one of the most famous stories is about two parents of a dying world putting their son in a spaceship and sending him to Earth.
All things come to pass, but we leave behind remaninants, legacies, hope, and so on.