The seventh season, which Whedon was much more involved in, reportedly, is a much bigger disaster, both storytelling-wise and in betraying some of the things the show stood for.
I actually agree that the seventh season of Buffy was worse than the sixth (the sixth had some terrible episodes, but the seventh season was stone-cold boring), but still, Marti Noxon was the showrunner for the seventh season as well, and she’s got to take blame for stuff like writing the “vampires can be drowned” episode.
But as he notes, if the creator of the show comes back and gets more involved, that doesn’t guarantee that the show will get better, and it may actually get worse. In the age when fans are actually aware of showrunners — used to be fans didn’t know the name of any executive producer other than Gene Roddenberry; now showrunners are minor celebrities — there’s a tendency for fans to connect a decline in quality to the absence or decreased involvement of the creator. It’s rarely that simple, because if creator has input into picking his/her successor, then he or she will probably wind up approving of the results.
So Simpsons fans spent the Mike Scully years lamenting that none of this would be happening if Matt Groening weren’t involved with Futurama and if the original writers were there, and then Futurama got canceled and Simpsons veteran Al Jean came back as showrunner, and the show eventually wound up having all the same problems over again. Because if Groening or James L. Brooks really hadn’t approved of the jockey-trolls stories, they’d have had the power to put a stop to them.