Right but who are they?
Andy Reid, who notoriously ran a superbowl contender for over a decade in Philidelphia. Who then went to Kansas City and got a better QB, and we see the results. You can try to debate that he's improved, but the simple truth is that he just landed a generational talent at QB vs in Philadelphia he only had a good QB. He was a superbowl contender in Philly, now he's a superbowl contender in Kansas City but winning them with a much better QB.
Dan Quinn, who turned Atlanta into a superbowl contender. Has now put together a superbowl contender in Washington. Only now, he has what so far appears to also be a generational talent on QB. Its reasonable to believe that as long as JD5 does not regress, its only a matter of time before Washington walks away from a season with a ring.
Let's go a step further.
Dennis Allen, who was one of the worst coaches in history with the Raiders, coached New Orleans and was still one of the worst coaches in history.
There's quite literally a plethora of other examples were a retread coach continues to succeed, or fail as they did in their previous position. Sean Payton is another example, Tony Dungy, John Gruden, the list quite literally goes on and on.
On the flip side of that, what about coaches who have pulled a complete 180 to some degree?
Most know about Bellichek, and there's a few others. But very few coaches fail early to succeed later in a 2nd chance. Then you have the coaches who pulled a 180 in the other way, who were successful but came flops and never bounced back. They're not very numerous either.
It tends to be that whatever retread coach you get, they are very likely to continue to be who they were in the prior head coaching position, and the only difference leading to more or less success is the players, particularly the QB position.
Therefore, it seems to reason that retread coaches are not bad at all if they have a history of superbowl contending. They are more likely to still be a superbowl contending head coach then not. Leaving the question of, does your franchise already have a superior QB talent, or do you believe that coach can find one. Because if not, they likely may not be as successful as they would've been, but still somewhat successful.