>>282156724
>Luffy and Ace's minds were set permanently by then
The only one whose mind was set was Luffy and that was because of Shanks, and to be fair to Garp, like Dragon, Luffy would probably leave the Marines even if he joined, Luffy wanted to change the world. All Ace needed was a family, and if Sabo left before the burning, he probably would have no reason to side with the Revs, like Ace he wanted a family too.
The problem with Garp's decision to just leave them in a island, is that it's something Luffy would do, but, unlike Luffy, he's not the protagonist so his decisions might backfire in a meaningful way. Oda does this a lot, with Garp, Ace and Kid (and probably some others that I'm not remembering right now), where they do something that Luffy would do, but they fail because of it.