The Ferrari theory is certainly an interesting one.
It does imply that his Williams contract was for two years only.
I think the 1994 Williams was just as good as the Benetton towards the end of the season, so he would’ve been challenging Schumacher in the championship.
1995 however, the car was not on the same level as the Benetton throughout the season.
My guess is that he would’ve had an ordinary 1995, and been looking for a drive at Ferrari, but they still would’ve gone with Schumacher because Senna would’ve been asking for crazy money, so Schumacher was cheaper and getting results.
Continuing with Williams would be unlikely, because he would’ve demanded more money, and he would be frustrated with the car.
I think he would’ve retired at the end of 1995.