I mean, at that point if would be just name changing, good/bad present instead of good/bad future (original present wouldn't exist). Which indeed does make it sense because, how come that the present doesn't get changed after you blow up Eggman's machines in the past? But then there is an issue with that, once you get all the time stones, you basically have no challenge at all? Since bad timeline wouldn't be no more, you'd just start in paradise land and... that's it. Yay. It's definetly not comparable with being Super on Sonic 2 because getting Super carries over its own set of risks, while good future would have no con at all. And we could say to remove the always good future from getting all stones, but then they would be equally pointless than S1's emeralds, gameplay wise. Which for me personally wouldn't be strictly bad, but with the deeper plot that Sonic CD has, it seems kinda dumb. Or, alternatively, getting all time stones set you at original present, and you can go to the future to still have the original rewards? But this isn't exactly the most consistent approach.