I think there's a few factors at play here, the first of which you described in the OP. But I genuinely don't think this is an issue of 'popularity.' Rather, it comes with just having been around for a while. I'm not a big creator, but I'm doing okay so far, so I'll speak from my own experience.
No need for promotion.
Once your series starts gaining momentum, promoting on a forum isn't really necessary. That said, this only means a creator who is doing well doesn't need the 'Promotions' section, of the forum and will probably mute it. That's what I did a while back. (Not that I promoted excessively, anyway.) But that alone doesn't mean I don't find the rest of the forum valuable.
Finding other communities.
My activity here on the forums didn't really start to dip until I found myself spending more and more time on Discord. I have a number of friends there now. I know their faces and their voices. And, because there's a finite amount of time in the day, I find myself sending my social time on Discord rather than here. That's the biggest reason, for me at least.
It has nothing to do with having an audience, though. Many of the creators I count as good friends have a smaller readership than I do, yet they've found themselves on Discord more and more as well.
Same topic, different author.
This is the other big one. After having been here for around nine months now, I'm definitely starting to notice that certain topics come up again and again. Having chimed in a few times in the past, there's just not much point is re-stating what I've already said for a third, a fourth, or a fifth time. So I don't respond, and those discussions carry on without me. This is becoming more and more common, to the point where I check in some days and there's nothing for me to actually say.
This would also be true regardless of whether my comic was doing well or not.
So, I don't really have an answer to your question specifically, as I don't think there's a particular 'popularity milestone' a creator reaches which causes them to engage here less. Rather, I think it's probably a question of time. For me, around nine months is when I noticed my activity here begin to taper off. I still check the forums frequently, but I engage far less, because I've already given my input on most of the topics which appear.