I believe the following is happening among hobby joggers
1 - It's marathon or bust when it comes to the roads. Fewer people care about other distances, more care about marathon, and have better info to better train for it. I think for this reason you may continue to see the BQ times improve as overall running participation decreases.
2 - local races are dying. There are still dozens every weekend in my region, but so many of them have only 40-80 participants and are won in 20 minutes. I think in the next few years we'll start seeing them being disbanded.
3 -ultra, trail races and spartan type races are growing, and more people training for them
In all, I think what running during the running boom offered people was a way to get in shape, and by hitting certain times or places was also a way to boost ego/show-off, no matter the distance. Now, no one is impressed by 5ks to half marathons anymore, no matter how fast you run them, so the participation is dying off. People are moving to sports where they can get fit but also show-off by doing something they think impresses people (like an ultra, marathon, BQ, spartan race), in most cases just by finishing one. A 25 minute 5ker figures they can do the equivalent effort in a spartan race or marathon but impress people (at least people who dont know better) while no one would be impressed by their 25 minute 5k. Basically it comes down to the boom is over because people are getting less self satisfaction of thinking they are impressing people with their 5k-hm finishes.