This is such a lame argument that gets thrown around all the time. Of course if you limit your thinking to well-established names who have been around the NFL for years and are also available for hire immediately, you are going to come up with zip. But that is just by being deliberately obtuse for the sake of proving a point.
Who were Belichick, Carroll, Reid, and Payton before they were successful NFL coaches? Were they already NFL head coaches with a 10+ year track record of success? No? Then how did they ever do it? It's almost as if there are other people out there who might turn into good head coaches if you hire them. In fact, you might say that's how every single successful NFL coach is found.
This whole line of thinking is no different from saying, "How are we ever going to find our next QB in the draft? Who's coming out of college this year that's had a better NFL career than Ben?" Completely rhetorical and disingenuous.