What does time stone have anything to do with time travel though? What I have seen so far is the time stone can reverse or fast forward time within a limited span and see multiple possibilities in the future. These are not the same as time travel as the GPS enabled time machine they invented in End Game. (Say if you are in NY in 2020, you can reverse to 2019 or 1973 in NY, if that’s how powerful the rewind is, but not to 1973 in Hanoi. You will still be in NY with your stone, even if you can rewind to 2000 BC). Whereas a space stone can allow him to go to any corner of the space and perhaps chance to get hold of something or someone that can enable him to travel back in time and targeted place. That’s just how I understand from the materials in the movies so far.