There are still plenty of knowledgeable people here. Questions still get answered. New players tend not to ask the questions that only the most knowledgeable can answer. They're more helpful with the deep weird behavior stuff that comes up from time to time.
And some of those who left tended to snipe at those asking what they considered simple questions. Which is not very welcoming.
So knowledge has been lost, but not the type that's useful to newcomers. And when the weird stuff comes up, we can go looking over there.
I'd be more concerned about the modding, except that there still seems to be enough interaction that I don't think much has really been lost. It's just a bit split up.
As for "long-term health", I'm aware that some people like that kind of environment and I'm fine with that. I'm sure the site will continue as long as they want it to. My experience with environments like that is that they tend to pare down to a in-group that it's very hard to break into. With few new members, the site dies as people leave for one reason or another.
I've usually seen this as a takeover of an existing site, not a splinter faction. It may well be in this situation, new people who prefer that environment can come here and build up enough cred playing or modding to get accepted over there. It's an interesting little social experiment. We'll see how it plays out.