I have a theory, though. Fanfiction is dying... because in many ways it went mainstream. Whether they call it "reboots" or "alternate universes," a large number of fandoms saw themselves reborn in book, TV, and movie form. As such, what was once a free offering online by dedicated fans became the new canon of "official" publication/release and the line between the original creation and additional fan contributions has been heavily blurred.
For me, I've always wrote Trixie stories for my own amusement. If people enjoy them, I'm very glad, but in the end, they serve as escapism for me and in that way, I'm actually glad the Trixie Belden series has mostly stayed under the radar where the copyright holder is (as far as I can tell) content to let sites like mine go unnoticed and unremarked.
That said, I do think TV Trixie would've kicked TV Nancy Drew's rear end, if she'd ever had the chance.