I'm not a 2003 tier oldfag, I was 6 years too late for that but I think this is plausible.
I don't think much if any of the original userbase still lurks 4/b/ for example since it was already irrecognisable when I started lurking, let alone in 2019.
I think the longer you've been there, the higher the chances you broaden your horizons and find new and interesting places. I doubt someone with even just 5 years of lurking stayed in one place for very long. Sure, I too check back on some themed boards on 4chan now and then but I won't do the full time lurking a la press F5 on page one every 5 minutes anymore. A least not there. That's why I think the original oldfags are long gone elsewhere.