The thing that has changed is that a huge swath of new people have come online and, though some of what brought them online is wider access to connectivity, a lot of what brought them online are new kinds of communities. They showed up for social media and most of them just aren't that interested in the things that made up the "old" internet.
I put "old" in quotes because people have kept and maintained the parts they love. You can still play MUDs, you can still visit BBSes, people still run Hotline servers[1]! Many of these communities have changed because the world has changed: lots of people who played MUDs in 1990 have moved on to other online games, but lots have not! Critically - tools have continued to be developed. You can use IRCCloud (and be told it makes you a bad IRC'er), you can play MUDs on your phone, etc. These communities have changed with the times and improved for it.
My sense is that the absolute number of people who are involved in these communities has dropped, but not actually by that much? Maybe half as many people play MUDs now as they did at the peak - but it's a steady half. I think of it like the communities around vinyl or around film photography: less central than they once were, but healthy and vital.
I am really glad that people who were not online at all during their peak are discovering these older forms. We have kept them for good reasons. But don't call it a comeback, they have been here for years.