It's probably a split between personnel and philosophy. Capers contemporaries seem to adjust with varying levels of talent. The Patriots have had revolving doors at numerous positions but their defense seems to manage. The numerous mental mistakes, blown assignments, and sloppy play point more to coaching than talent.
At the same, they seemed to over correct heavily this year. McCarthy and Capers "wanted" to get faster and leaner along the line. So they dumped the big uglies up front except for Raji and now they'll get knocked around by power teams, especially on fast tracks.