My personal theory of Weezer is that some time around 2004, some hipster who spent high school moping to Pinkerton killed Rivers Cuomo's dog, and Make Believe, the Red Album, Raditude, and now Hurley are all part of an Ahab-esque quest for vengeance. Alternately, we can embrace the Matt Sharp theory.
The correlation is pretty compelling. Weezer releases two of the best rock albums of the '90s. Then it breaks up. It gets back together without Sharp as bassist, and soon enough "We Are All on Drugs" happens. But what this ignores is that the Green Album and Maladroit, while certainly not up to par with the first two records, are still very fun. I'm not going to say that "Photograph" is as good as "Why Bother?" or something similarly ridiculous, but it, "Hash Pipe", and "Island in the Sun" are legitimately good singles. "Dope Nose" holds up well, and "Keep Fishin'", if a little saccharine, is as catchy as anything the band ever wrote:
I'm willing to accept a limited Matt Sharp theory, which posits that the band's decline from all-time great status to being merely quite good was due to his departure. But the band only became a true horror show after two albums without him, which suggests some other causal mechanism is needed to explain the full scale of its downward spiral.