I do think it's a combo of things and you're right that athleticism does progress over time. I guess my thinking is that in a playing field that isn't getting any larger, there is only so much room on the field. Yes, the offensive athletes are even better, but that doesn't make the playing surface larger.
It's a distinct advantage for the defense to narrow down/shrink the room the offense has to operate. The offense has no such counter now that they've maxed out the vertical spacing they can achieve. (That's why I think the late 90s, early 2000s saw a huge jump. Vertical spacing became a huge factor. Defenses have now largely eliminated that with 2 deep looks while still having enough athleticism to shrink the intermediate and shallow areas of the field)
It's my explanation for the pivot to jumbo sets. Or LA running 4 tight ends. They know they can no longer stretch the field as an advantage, so they're going to rely on winning by pure size instead. There will be more ebbs and flows, but I think one thing that will be here to stay is that defenses are going to be much, much more difficult to attack because they can simply defend far more of the field, far faster, than they ever have.
The only solution (and I'd wager we start hearing about it soon) is to widen the playing surface.