Who's to say after obtaining the Soul stone & in his inventory he can't bring Gamora back. I mean he's dang strong now. He /did/ bring all those Eternals back.
If im not mistaken the soul stone gives you power over life and death. In my mind. . . that makes it the most powerful and most precious of all of them. Something that honestly only the true God should have. Noone else can fully be trusted with it.