I also check the site often...at least a few times per week. It doesn't take long since the nice feature that displays current posts shows how utterly inactive it's become. It was pointed out that the look/feel of a changeover put some people off. I think it's a combination of a few things.
I don't think the look/feel was the biggest problem. That it took so long to transition was a bigger issue. Visiting forums...and having conversations is a habit. That's why FB and phones are "addictive". When you remove an addiction, it's easier to break the habit. Maybe "having a different look/feel" is a polite way for saying the site was delivered "broken" after such a long hiatus? Functions don't all work after that transition and (to me) links to IMPORTANT historical threads weren't retained. Tracing your way through what USED TO BE some great information is now MUCH more difficult. It's sad that the site owner cites major health problems for the inability to restore better operation. I would add that the passing of time aggravates any ability to restore/relink historical posts/information.
Ironically, one of the last posts (by the owner) suggested he was looking for a buy-out. (That's how I interpreted his feedback anyway). Like a fine sports-car, if you tear into it and "break it" before the sale, the value to others is hurt. So, it doesn't seem likely that will happen. As a profit enterprise, you also have to consider the size of the audience -- which seems also "damaged" ...especially the longer site inactivity continues to be the norm.
Really, the older our vehicles become, the more likely I would think it would be to seek out assistance to diagnose, fix problems, and source parts. As such, I'm not sure you can point dwindling participation to much other than site problems.
2001 Ebony VX and 1989 Custom 383 Corvette