You're not wrong, but it also depends on why a person chooses to be on a forum. If one is only in it for themselves, yeah, they probably wouldn't post to forums after they hit a particular level of popularity. But I joined the forums here not only to promote my comic (though that was one reason) but also because I wanted to be involved in a community with other webcomic creators. I'm a slightly more experienced artist than the younger creators here, and I thought I could offer some good advice, feedback, wisdom, and whatever else.
I can't see that changing if I ever got huge. There are only two reasons I could see myself posting on the forums less, and those would be lack of time, or feeling as though I were no longer welcome.