Agreed. There have been six Defensive Coordinators in the past 10 years:
Perry Fewell, 2006–2009** (under Jauron)
George Edwards, 2010–11 (under Gailey)
Dave Wannstedt, 2012 (under Gailey)
Mike Pettine, 2013 (under Marrone)
Jim Schwartz, 2014 (under Marrone)
Dennis Thurman, 2015-present (under Ryan)
Not only that, it hurts the defense even more when each coordinator brings a different style (3-4 vs. 4-3 vs. wide 9) and expects the players to fit in and play. Then that begets changing the players who don't fit in and then the job is never completed where the defensive personnel fit the scheme.
A lot of that pendulum-swinging can be laid at the feet of upper management who could have and probably should have insisted that each new coordinator bring in the same scheme as his predecessor. There are enough coaching candidates each offseason to get that accomplished each time the team needs a change. At a minimum the team should choose a style of play and then stick with it - no matter who the coach eventually is. Major changes should only be made when absolutely necessary IMO.