In those folks cases though, weren't they always pure sprinters? I think it's a little different for a middle distance guy to move down to the sprints in his 30's.
If anyone can do it, Phelps can. It is risky though, and I'm not sure he would want to come back just to swim the #2 leg on the 4x100 and 4x200 relays? To swim on the medley relay, you pretty much have to win trials, his only shot there is the 100 fly (which he can do, of course). He doesn't quite have the speed in the 100 free to be top 2 (at least not historically, but if he focuses on the 100, who knows...)
His real strength is fly and IM. I think he will gravitate back to those, if he really wants more medals. If he dedicates himself, he's pretty much a lock to make the team for the IMs. It's just a matter of how much pain and suffering he still is willing to put up with.
Swimming Workout of the Day: Favourite Swim Sets: 2020 National Masters Champion - M50-54 - 50m Butterfly