I agree. If there's one thing that's true about the NFL, it's that the defensive coordinators in this league will ALWAYS catch up sooner or later no matter how much an offense innovates.
I think of it like this: offenses are constantly trying to evolve and surprise opposing defenses with new innovations, but much of it is smoke and mirrors. Defenses don't really innovate regularly, in so much as they are instead actually responding/reacting to offensive innovations and shutting them down.
On the other hand, true defensive innovations are rare (such as the concept of zone coverage, or the zone blitz, or maybe even two down linemen a la Super Bowl XXV). True defensive innovations tend to be far more impactful on the game of football and they spark a whole slew of staple offensive plays and/or formations that are designed to counter them (for example, Joe Gibbs said the Redskins created the two-TE base package as a direct response to the Giant's pass rushing via Lawrence Taylor).