He's already relegated to staying there, unless he is willing to go somewhere even less competitive, or leave MotoGP. There's Honda and Yamaha. Then there's Ducati. And then there's the also-rans and CRTs. That's it. He has already been at both Honda and Yamaha, and neither will take him again. So his only hope of winning anything in MotoGP again is for Ducati to figure out how to make their bike competitive, to pick up that last 1 second per lap.
The only scenario that changes this is if Ezpeleta makes some sort of deal or decree to make Honda or Yamaha take Rossi back. In which case, even the pretense of it being a real racing series goes away, and it's no more real than a kid in the sandbox waving his toys around and making vroom-vroom noises and deciding the blue bike wins this race because he likes it best.
Indeed, Ezpeleta's proclamation that Rossi will be on a competitive bike next season I find to be very worrying. If he steps in and further interferes with who teams hire, and when and for how much and all that, the entire series will be done as far as I'm concerned, until that fuckwit is fired and someone takes over that will make a sensible set of rules, make them stable, and get rid of as much "spec" stuff as possible (starting with the tires).
PhilB