I really like your explanations in the second paragraph, especially regarding Crimson, you summed up very succinctly the smart moves Fripp made.
I often have discussions with old rock-heads who bemoan the lack or loss of success by some of the sorts of bands mentioned above, but as you rightly said, these are the bands that largely didn't alter their sound. The rock heads describe this as a good thing and that those bands "at least didn't sell out" etc etc. These are generally the same guys who think that's what Genesis did i.e. "sell out" and that nothing post-1980 is worth listening to. One guy said to me "they sacrificed their musical integrity", something which no-one who ever saw them play live could ever reasonably say. My view is they maintained that integrity, but just applied it in ways those guys didn't like.
Yes seemed to be on the decline but with the arrival of Rabin and making 90125 they plugged in to the boom in FM-friendly shiny rock and they took off all over again.
Tull is an odd one isn't it. I was about to say, did they really have sustained success but then checked their album sales and they were having gold or silver certifications in the US and UK right through to Rock island in 1989 which was silver in the UK. I'm not familiar with their work from the late 70s onwards but if as I surmise their style didn't really change much at all, then yeah they buck the trend of stick-in-the-mud bands withering and dying.