I'm sure Rumsfeld and Cheney could find somebody who would give the orders they want. However, it appears they would have to cut through several layers of command staff and would end up with some totally unknown career staff guy.
Faced with mass defection or removal of the senior staff, I expect the response of the majority of mid level officers would be to order their men back to the barracks until the situation is over.
A much more credible danger is that the neocon cabal around Bush will pick another puppet to replace him. There has been a lot of talk of Cheney stepping aside at some point to let the next annointed one get a head start on the next election. But Bush is so unpopular right now that would be more of a hindrance then a help. More over, as Iraq implodes the entire cabal is coming apart.
Jay