I don't know - when Bowie's bad he's pretty bad, but he's kept up to date with current trends (often predating those trends by about 20 years - I mean, the mid-90s bands all sounded like they were influenced by early 70s Bowie).
I also disagree with your assessment that all of the Rolling Stones's albums since 1969 have been bad. Tattoo You is one of their best, and that came out when, 1981?
I notice more bands that have really great first albums and then lousy second albums. The Black Crowes come immediately to mind, although I'm sure I could cite other examples. The Stone Roses.
Of course, bands usually have years to work on songs for their first album, and then a year or two after that are expected to crank out another one, while in the meantime they've done nothing but tour playing songs from the first album.
Then again, there are a number of bands whose second albums were better than the first - Neil Young, Dylan, Pearl Jam, Fiona Apple, Ben Folds Five, Soul Coughing, Morphine - and then those who seemed to just get better and better before some ill-fated collapse - The Beatles, Nirvana, The Who, Led Zeppelin...
The only band that used to be good and now is awful that I can think of off the top of my head is Counting Crows. The past two R.E.M. albums have been pretty bad, but they're so prolific that two albums almost doesn't represent a trend. I'm like the kid whose parents got divorced, thinkng that Bill Berry will come back and they'll make one final album together as a band and it will be their best album ever.