It's hard to compare eras because of the increasing athleticism (read: performance enhancing drug use), but within an era, there have been nearly bullet proof teams in every decade except for the last two. And the 2000s Patriots are still better than anyone is going to be this decade, because the level of competition at the top continues to deteriorate.
edit: Another possible contributor is the pass happy, pinball machine garbage that passes for football today. Defenses get worn out because passing slows the clock down, which may turn what would have been a decent defense in other eras into a much worse one today.

