Personally, Coyle would be toward the bottom of my list of guys to replace. People are getting a bit revisionist about him and maybe forget how stingy we've been in allowing points until recently.
What's missing in the D are aggressiveness in tackling and in coverage. Part of that is scheme driven, where Coyle has to take some rap. But I also think we took Finnegan and Delmas to be the fiery motivational guys who the D rallies around. When the head coach drops the guy who's supposed to be the lightening rod for the D, don't be surprised if things go a bit flat.
Maybe having the DEs cover TEs wasn't great, but it was partly injury led and nobody was calling Coyle a bum when his 5-2 alignment destroyed NE.