2014 Thanos can't sacrifice Gamora, because he was also stolen from that timeline
Endgame established that changing the past would not change their past, it would only create a new timeline. In the new timeline where Gamora was taken to the future, Thanos was also taken and then dusted. So for that new timeline, Thanos will never complete his quest or need the soul stone. This is one change to the timeline which is unavoidable, but also manages to save a second timeline from the snap in the process.
However, for lots of other things to happen correctly in that timeline, the stones must be returned. The main one pointed to by the movie is the time stone being used by Doctor Strange to stop Dormamu. However, there are many other things they would want to happen that require returning the stones:
Without the tesseract from 1970 onwards:
- Captain Marvel doesn't get her powers
- The Kree/Skrull war isn't ended
- The Kree intelligence isn't defeated by Captain Marvel
- Any other feats in future Captain Marvel movies
- Nick Fury isn't awakened to the wider world
- The Avengers Initiative isn't started
- Loki wouldn't need to attack Earth, which also prevents the Avengers from forming.
- No staff brought to earth, which causes all of the following
Without the Mind Stone being returned:
- Age Of Ultron doesn't happen
- Vision is never created
Without the Power Stone being returned:
- The Guardians of the Galaxy never form
Without the Reality Stone being returned
- Whatever happened in the end of Thor: The Dark World wouldn't come to pass?
I think for the sake of that/those timelines they had to return them, or risk sacrificing another universe for their own.