I'd have to disagree with you here. Linus can't prevent forks because of the nature of the GPL. There are already kernel forks out there. Some are actually fairly popular, as they are branches for developing/proving new ideas that aren't ready for prime time. IIRC, Alan Cox's kernel is used for this purpose. There have been several ideas that have been more fully developed elsewhere because Linus didn't see them as 'good' until they'd been more fully developed elswhere, which were then later merged back into the main tree.
On the other hand, Linus is in a rather uniq position as 'benevolent dictator' of the main Linux branch. Though not always liked, he's generally respected, which is that allows him to operate as he does. Without Linus, it would be much harder to find someone who could get the same kind of respect and cooperation. In that way, he's pretty much irreplacable at the moment. Who could be the new Linus? I really don't know of anyone who could herd cats as effectively as he can just by virtue of his position.
Yup. He carries a lot of weight and what’s more he’s usually right.