No car manufacturer is going to spend money on an engine making it better than the Cosworth option only to have it limited. Especially seen as the major issue with car manufacturers is the cost of development just like this.
Basically, the Cosworth engine needs to be utterly epic to make the manufacturers want to keep up (which, lets all be honest, isnt going to happen, especially seen as they will be getting a comparitive pittance from each customer, so no money to develp the engine). So this means all the manufacturers are going to be like... uh, whats the point...
Look at it this way. This ruling will force all teams to be equal, but that the term 'equal' is actually defined as 'no faster than the slowest'. Basically, the rulling says that no team can be faster than Force India, and if you are, they will restrict you. Whats the point in a car manufacturer investing in that? They will just go elsewhere. After all, the car manufacturers are in F1 to promote their brand, and they cant do that when they arent allowed to be better than their competitors. They wil just go elsewjere where they can develop cars to be the best.
And Malth Viniger - Whats the difference between being forced to use a standard engine, and being forced to make your designs the same as a standard engine? there simply is no option availoable to the teams that will allow indipendant development.
EDIT: I feel I must add, however, that if there was a long list of privateer teams desperately trying to join F1 and build there own chassis and produce some radically different designs, then I wouldnt have any issue with them having standardised parts. Maybe not the entire drivechain, but the engine on its own maybe. Afterall, there is an awefull log on an F1 car that is standardised already. But the fact of the matter is, there isnt a list of people clambering to join F1. Even those who do fight there way in (Prodrive) are prevented from joining through conflicts between teams and badly managed decisions. If Jordan, Minardi, Prodrive, and a load more private team owners suddelny said they want in, and the FIA stated that they would allow customer chassis and a standardised engine design (not a physical unit) then I would be all for it. A mish mash of engines matched with different chassis, and again on 2 or more tyre suppliers with teams of privateers who are in it for the winning and nothign else, I would love it.
The standardised engine concept has its valid good points, but in the current situation in F1, wherer the majority of the grid are made up of car manufacturers, it simply wont work.