But with respect ovi, you are going for a best case scenario with the Scott Walker example - one of the rare exceptions that proves the rule. Even then, "Tilt" was just one album, it's not as if he was still pumping out a series of consistently strong works which outshone the music he was making twenty five years earlier. The original point obs was making - which I agree with - is that most multi-decade spanning acts release their best work within the first ten years of that career and on the whole I suspect that's true. Sure there are exceptions and your point about acts being commendable for maintaining critical and commercial success (rather than just sticking together or still touring) over a long period is well made, although quite why Springsteen garnered either, even at the start of his career, will forever elude me personally.
I still feel that this continued commercial (and even sometimes critical) success is maintained over decades through habit/familiarity. This idea that just by "still going" decades after an act first hit the scene, whilst not being trumpeted by you, is nevertheless held in bafflingly high regard by the public in general - and in the context of rock/pop and its original "live fast die young" premise, as obs says, it goes completely against the grain. It's the same mentality that awards "lifetime achievement" awards to TV stars and actors, basically just for staying alive and not retiring. The Stones are the obvious example, stale, old, trapped in a cycle, playing safe and lacking the individual talent and the professional courage to split, grow and develop. But it's The Stones, and since they've been around forever and are no longer a raw, dangerous threat, they are now "National Treasures". Yuck!!