They shouldn't have brought them back... (Spoilers)

The way I understood it from the Ancient One's explanation, there was only one timeline (indicated by her one long yellow line). But the moment you try to change something in that timeline then it will break off into an alternate reality. So the reason why they wanted to give back those stones at the exact moment they took them was to make it seem like nothing changed at that moment, therefore not resulting in an alternate timeline.

But by bringing all those stones from the past back into the present then reviving half the universe, a present where the stones were already supposed to have been destroyed, they changed their own timeline which would have resulted in them breaking off into an alternate reality. So they only saved their own alternate reality. The main timeline still ends up with half the universe dying and by bringing back the stones to their respective time zones they guaranteed those timezones all stick to the main timline and end up in the Snappening.