I don't think that Sonic's final smash needs to be changed to his previous final smash. What I think should happen is that whoever is hit by Super Sonic should be knocked horizontally and at a downward angle (like Zelda's downsmash) instead of popped up. I could see Sonic's new final smash turning into one of those ults that serve as an edge-guard. Instead of taking opponents off the top, one could wait until one or more people are off the stage, activate the final smash, and let Super Sonic act as a barrier that keeps opponents from returning to the stage. Maybe something like that would make Super Sonic more viable but still keep it short and sweet like the developers want all of the final smashes. Rosalina's final smash seems to be an edge-guard too as she can wait until opponents are off the stage, run off it herself, and place the Grand Star right next to the ledge.