PC as coach is not the problem. For all his shortcomings as an in-game coach, he more than makes up for it with the positive winning culture he creates around the team.
The main problem has been talent acquisition. Excepting last year, they've drafted poorly for some years now. They also spend too much CAP space on middling players, rather than impact players. And, of course, the Jamal Adams trade was horrible in retrospect.
The real question is who has been the lead dog when it comes to making personnel moves? If it's been PC, then he's had a pretty poor record for a number of years, excepting last year.