It's not just the mixing of coverages though. There's only so many variations that they can employ in game. Back when I played, we used to play split field coverages like 20 years ago, the NFL didn't use to coach basics. It was the assumption that you knew how to play a zone properly when they drafted you so they'd call a cover 2 and expect the players to understand how pass constructs work and to never just guard grass for no reason.
A straight cover 3, undisguised, coached and played correctly, is one of the most difficult defenses to complete a pass against. That's what Pete did at Seattle for years with Sherm, Earl, and Browner. I think the DCs in the NFL is where the experience lies with coaches right now and some of the better ones are actually taking time to explain how to understand where to be when they're in a zone and when you should evacuate a zone to help a teammate who's zone is being flooded. It's an extra bonus that almost all of the OCs in the NFL come from the same offensive tree so you don't have to learn new concepts every week like you might have to do in college. You just have to learn tendencies and personnel, which is easier, except for the special ones (Lamar, Josh, JJ, Chase, Pat, Saquon, etc.)
The bolded is an angle that I hadn't considered, but actually seems obvious now that you mention it. I haven't actually checked, but it does seem like DCs are tending to be longer tenured now, whether they hold the position longer, or have been within the organization longer, and worked their way up. Being able to keep the same philosophical identity long enough to truly establish a type of player that the front office needs to get them, along with a defacto SOP of how to teach concepts out really does go a long way.


