Seems gravedigging isn't frowned upon on this forum, so I thought I'd add my view. :)

In my opinion, post-Waters Pink Floyd is just as empty as post-Pink Floyd Waters. Roger tried to work with all kinds of musicians (because, admittedly, Roger is not a musician. He's a composer/artist/whatever you want to call it. He creates music, but doesn't really play it), and it ended up rather horribly because he didn't have a sound of his own. Each of his albums sound completely different (hell, even each of his songs do) and there's no structure. Lyrically and compositionally, he was God, but he didn't have any arrangements.

About David and what was left of Pink Floyd; they had arrangements. They had a great sound (I think TDB is even as good as Dark Side of the Moon, sound-wise) but they didn't have any songs or inspiration. And in case they didn't have the "Pink Floyd" name, they would have been just as unsuccessful as Roger was on his own. I know it may sound degrading, but David was clearly not made for leading a band on his own. When I listen to his songs, all I hear is an overuse of guitars, and half-assed lyrics with amazing one-liners but not a single song of which each line is spot on. (High Hopes may be an exception to this.) His attempts to make his music more "special" were vain, too, and that's something Roger completely failed at as well. The fair-like sounds in Poles Apart and the woodcutting sounds in one of the songs on Amused To Death (can't remember which) are completely useless and meaningless. I'm quite sure they were both just trying to sound like Pink Floyd, though.

All in all, it's quite simple. Both tried to do things without each other, and both failed. They had always relied on each other, and they simply couldn't create any decent music without their usual bandmates. It's the fate of all great artists. In case Roger, David, Rick, Nick or Syd would not have existed, the other four would never ever have been famous. (With the exception of David, I guess, since he's an amazing guitar player who could probably fit into any band.) Syd's departure was a different story, but when Roger left, they weren't going to be as successful/influential as before. Simply impossible. Even Lennon and McCartney failed at that; how could Pink Floyd possibly think they'd succeed? ;D