Final decisions are made by totally independent folks like Linus, Greg or Andrew. They set the rules and also the community, corporate has to adapt to them or go away.

Also, what happens is that rewrites happen progressively, in small steps.

Not, I think, in the sense meant here. You're right, of course, but the distinguishing factor is that "corporate software" typically only involves a single corporation. There's no (or little) balancing of interests.