People didn't go to what.cd and jump through all those hoops to find rare music, they went to find good music, and now Spotify and YouTube do a good enough job of that with a lot less effort.
As a funny example of that I discovered Billie Eilish when Ocean Eyes first came out from a private tracker and now she's about as big as anyone. So it isn't just about discovering rare czech folk singers, but any music that you might like.
I think the hardcore demographic you mentioned is spot on and does exist, but they were always a minority and the evaporation of the less hardcore users explains why the scene is so much smaller now.
It does because private trackers use buffer as currency. The music trackers have had to adapt to use points systems now to encourage downloading activity. But even still, activity is way down from the what.cd days. It's just easier to stream the easily accessible stuff.