I think it was Steve Hackett leaving that was the cause of Genesis’s decline
Ironically, that was the example I had thought of when seeing the thread
The Beatles, after Paul McCartney. 
He was the leader of the band, but it was a little more complicated than “they declined after him.” They achieved their greatest fame and influence (though they were no longer as intense or creative) after Duane, when “Ramblin’ Man” was a big hit and they did (one of?) the first stadium tours. After they lost Berry Oakley in 1972, the ‘original’ band was kind of doomed. I think they’re still doing some excellent stuff today, but that’s not really the band that history talks about.
Oops, I think I misunderstood the OP, I took “declined” to mean “declined to continue as a band”, not that the quality of the band’s music declines.
They did. “Plays Metallica by Four Cellos” features Enter Sandman and The Unforgiven from the Black Albulm.
Squeeze never bounced back to top quality after the initial departure of Jools Holland.
I came in just to mention Weezer. I listened to Make Believe once and haven’t touched it since. It fully deserves its Pitchfork review of 0.4.
Mike never left the band. He did that Mike + The Mechanics thing in the 80s, but returned to Genesis each time.
It’s debateable, but I think that Depeche Mode just hasn’t been the same since Alan Wilder left in 1995. It just became that much more obvious that he was the one really creating the mood and feel of each album, not to mention remixing and reprogramming a lot of good old songs for the concerts. The new album is the only one that comes close to where Alan used to take the sound, but even now it’s not the same.
I’d say that Ozzy Osbourne’s solo career became much more ordinary after the death of Randy Rhodes
And pokey… I agree with you whole heartedly. I saw Bob Wiseman perform just recently, and it was great. I have no interest in seeing Blue Rodeo again.
I stand corrected. I was never a huge Genesis fan, and only have vague recollections about the early 80’s. Not due to age - I’m 32.
It happened to Pink Floyd twice
Then there’s that Velvet Underground album that was recorded after Lou Reed left.
Black Sabbath.
I would think then you haven’t heard the Green album, which has songs that should have been big hits. Maladroit also has about 6 songs that I actually like consistently, too.
Its strange how guys with little discernible talent, like Bez from the Happy Mondays, sometimes hold the whole show together. Take Queens of the Stone Age, for example, Josh Homme sings, plays guitar and is the general creative force of the outfit. Nick Oliveri handles the shouting, class A drug consumption and getting your johnson out on stage duties. Yet when Nick left earlier this year, their subsequent album was noticeably poorer for his absence 
I’ll say the Who after the death of Keith Moon.
The Rolling Stones also after the death of Brian Jones.
Mick Taylor was a great addition, but Brian really added to the Stones.