A few of the bands mentioned in this thread I've never even heard of. I have more homework to do, I guess. That's one thing I love about AK! :thmbsp:
I have mixed feelings about one or two groups mentioned. The Beatles, for example, got tighter (in both senses of the word, I believe!

) and better produced, more polished as time passed... but I think for the time, their early works were actually more consistently good. In the later era, they were heading off in various exploratory directions, which you might say made them more innovative, but often they were disappointing experiments, interspersed with moments of musical brilliance. I can listen to any of their early LPs, and while the edges may sometimes sound a bit ragged, the toe-tapping musicality is there. On some of their later LPs, I want to skip every other song. I'm not sure if, overall, I'd call that getting better, even if I can see (hear) the argument that says it was. I'd call it more of a trade-off.
Groups and performers that got worse or stayed the same are legion, IMO. The best LP Elton John ever did was his first one (Eponymous, with his head against a black background). Ditto for Cat Stevens (Tea for the Tillerman). Steppenwolf peaked with their second LP, although they continued --and even continue!-- to turn out some good stuff.
I suspect that in many cases, bands spent a long time working in obscurity to make their first good LP, released it to acclaim and then rushed later ones, under pressure from their labels to cash in on the popularity of the first successful ones, before the audience appeal faded. The result is that second (and subsequent) LPs are often worse than the early ones. The only groups that could resist this somewhat were the mega-performers who were worth SO much that they could somewhat dictate their schedules, and the record labels didn't dare to say too much. Just speculating here; I may be wrong about this.
But bands that consistently got better? :headscrat Hhhmmm, I have to go flip through my records and think about this...