The idea that because one company becomes so successful selling it's wares to various cellphone makers, that this means no one can buy that company, is absurd. If every cell phone maker allows themselves to be cornered like that, with no other option to turn to for processors, that is their mistake. Other options do and will exist.
There is no reason why another company can't buy ARM.
The problems start when a company such as Apple buys ARM and starts blocking the licensing of ARM processors to other companies allowing Apple to gain an unfair advantage in the market place over it's competition.