Actually I wouldn't mind it. I think Rex can be a good head coach in the league. I think he has to learn from his first job as Jets coach. You can be bold, brash, arrogant, loud, etc. if your winning, but the second you don't people aren't going to cut you any slack. The main problem with Rex and the Jets is the failure to develop Sanchez. If Sanchez were a slightly below average QB the Jets in all likelihood would be a playoff team.
That said as a purely defensive coordinator I wouldn't mind Rex. If he doesn't have to focus on making offensive personal decisions he could be very good. He (and the Jets GM) have not been able to assemble any resemblance of an offense. FWIW I think his idiot brother Rob is worse than him.