Not necessary IMO. Schwarzenegger is not vital to the success of the Terminator franchise. I quite enjoyed Terminator: Salvation honestly. If it wasn't re-written to appease Bale, and not given a PG-13 rating, it definitely could've been better.
But having said that, I don't think Arnold absolutely NEEDS to be in another one. If he were, I'd probably still see it.... But he comes off as a bit fuckin' full of himself in that small article you posted there, BG.
I think he seemed to "missed the boat" when it came to having a political career!