Gruden didn't develop a team into a super bowl champion...they were already made. They had been to 2 or 3 NFC championship games before he ever got there. Cowher has never been in charge of personnel decisions, who's to say he will be successful? That's as risky as finding that diamond in the rough. Gruden already proved he can't do a good job with personnel decisions.
Gruden also did a completely terrible job of doing any kind of talent development whatsoever. His draft record makes Ireland look great. He brought in a collection of veteran free agents each year, which every other year were able to make his team's competitive. But the core of that team was always what was there before he got there, and hardly added a single good player that was around for more than 1-2 years to that corps.
Gruden was good at adding pieces to something which was already there, that's how he got there in 2002. But asking him to build is like asking Rex Ryan not to eat.