I agree that having an integrated OS control every random co-processor in a machine would be undesirable, and that hardware abstractions are a good thing. But

I agree that having an integrated OS control every random co-processor in a machine would be undesirable, and that hardware abstractions are a good thing. But co-processors that can break OS' assumptions (regarding security, for example) sound like they should be under OS control. Not that this means under control of a single kernel but, at least, under control of some set of components that are developed together.