Maybe. But he also probably won't miss the playoffs half the seasons he coaches. Cowher went to the playoffs in 10 of 15 seasons.
If Reese stays on, Cowher is pretty much the only guy to do the job. He won with Maddox, Tomczak, and Kordell Stewart as his QBs... it's tough to imagine him not winning with Eli. Plus he turns water into wine and shits vanilla ice cream.