I believe he probably wouldn't, since by all accounts it looks like cares about the success of his pool, the miners involved in it, and Bitcoin as a whole.

But there's also the democratic factor.  Even if he has 50% of the hash generation, that's only as long as the miners in deepbit are willing to GIVE him 50% of the hash power.  Anything "weird" happening in deepbit or any other pool, and it's very easy for a huge percentage of the miners to go somewhere else.