According to the proposal from FIA the teams have three options:

1: Use the Cosworth powerplant
2: Build a new engine similar to the Cosworth spec version (actually it would even be based on the Cosworth blueprints)
3: Use their existing engine but with no development etc.

This would probably mean that the Cosworth engine will be just as powerfull as any of the other options. Mercedes, Ferrari and Renault would maybe prefer to go for option 2 (or possibly 3 depending on the competetiveness of that option) so that they could still claim to have their own engine (external appearance could also be different, which could visually differentiate the different engine manufacturers).

Another good argument would off course be cost for the Cosworth engine. I would guess that Ferrari, Mercedes, Renault Toyota and BMW would still build their own engine but apart from those, the rest would choose Cosworth.