This thought crossed my mind the other day. I got into geocaching in 2017, but even since then, I’ve noticed there are generally fewer new caches in my area, though the Pandemic seems to have boosted it somewhat. I would say there has been a decrease in popularity but the extent to which that is true is debatable.
One factor I have to wonder about is whether the rise of micros and nanos discourage new cachers. In my experience, those are the least fun type of geocache for those who aren’t already in the hobby to look for and find. But, it might not have much of an effect either. My experience is merely anecdotal.