To me, the question is more than just loss of interest in audio fora but engagement in the hobbyist aspects themselves. Long before the Internet, I had periods of intense involvement in the hobby, through such things as the NY Audio Society (or whatever it was called- the original publisher of The Audio Voice). A bunch of characters, some reviewers, ardent hobbyists, record collectors. Great fun.
Yet I went through periods where I didn’t even set up a system (though I kept the gear and all the records). It isn’t that I lost interest, but had other things, including work, that took most of my time and energy.
Once the Internet emerged, I got involved in it first as a lawyer- it was largely bulletin boards on fairly bandwidth limited ISP intermediaries. You’d read threads in a string. Over the years, I did some behind the scenes work for one hobbyist board (not audio) and a ton of work on Internet related litigation.
My experience also led me to get involved as a participant in a variety of boards, involving audio, cars, motorcycles, and other hobbyist interests. What I saw over the long haul was that participants change, old members drop off for any number of reasons, new participants join and the culture changes- not making a value judgment on that- it’s the nature of the beast. The topics change too as different technologies come into play- look how much bandwidth is now devoted to "streaming."
I think when you are in acquisition and evaluation mode, you are more likely to engage. At a certain point, at least for me, I’ve seen a lot of the same questions arise and don’t feel compelled to chime in; the topics are usually well covered. It isn’t exactly "lack of interest" but more where I think I can contribute something of value.
The social aspect of the fora also mark a change from the old days and I’m not sure we can go back-- there was something special about in person group listening sessions. Yeah, not all of it was serious and productive- a fair about of kibitzing, record swaps and just plain socializing. In some ways I miss that.
But the Internet opened up the whole world to us-- from record shopping worldwide, to communicating with groups of audiophiles from all over.
Trade-offs to be sure. I’m now about 53 years deep into this hobby and seem way more comfortable not knowing all the answers. So I still visit, still peruse the threads and chime in when I find it appropriate.
I have made many friends over the years through various hobbyist fora and for that I’m grateful even in cases where I no longer actively participate in a particular forum.