It took me a while but I think I’ve got it now. It may have been obvious to a lot of people but I’m guessing you’re not one of them else you wouldn’t be here.
So here it is in fourteen words:
In the Marvel Universe you can create alternative timelines but you can’t delete them.
In Back to the Future going back in time and changing things can send things off in a different direction like with Biff and his casino but it can also prevent things ever happening like people being born. In Endgame going back and killing a past version of Thanos would save half the world from obliteration in one timeline but when you went back they’d still be dead in yours. (This effectively happens because Thanos is killed before he lives the events of Infinity War.) So there was no saving anyone who wasn’t killed by the snap, sorry Vision, sorry Loki, but getting hold of the stones and reversing, not undoing, the snap fixed everything nicely. Of course there may now be a reality where Loki is alive, because the gang’s little trip to 2012 did alter a few things, but like Gamora is a different Gamora without the same experiences he’d be a different Loki.
With all of this considering then not returning the stones to where they snatched them from wouldn’t have made any difference to these people but this was a damage limitation issue. Put them back and you create fewer alternate realities.
The only way of undoing things is to literally wind time backwards which the Time Stone allows you to do but winding everything back five years would have taken five years which isn’t really feasible because presumably you have to concentrate on that task the entire time. It might be that you need the Time Stone to have been in your reality when what you are rewinding happened too so this is another reason why this plan wasn’t an option.
The thing I’ve not fully comprehended yet is how Scott Lang was turned into a baby but Tony’s explanation about how the time change was running through him rather than around him satisfied me at the time so I’ll not overthink it. I mean half of all life in the universe has been missing for five years and now they are suddenly back so what is that going to do society and the readjusted economy. That’s a lot of people with a lot of Games of Thrones to catch up with. You can’t worry about this stuff too much.
For a much smarter analysis of all of this click here.