During Endgame, the Avengers find a method of time travel, and use it to
recover the Infinity Stones so they can undo Thanos' snap.
During the movie, their ability to time travel this way is strictly limited by
their limited supply of Pym particles and inability to acquire more, due to Dr. Pym's death by snap.
However, by the end of the movie
the snap has been undone, returning Dr. Pym and presumably providing an additional supply of Pym particles (how else would Steve Rogers have been able to return the Infinity Stones to the timeline points they were retrieved from?).
With that limitation removed, what, if anything, prevents the Avengers from solving any and all problems that may come up in post-Endgame movies with time travel?
To clarify, the presence of
at the end of the movie indicates that at the very least individuals (heroes, their loved ones, etc.) can be saved from things that definitely killed them in the past.