All good artists have a finite creative period when they are actually worth paying attention to.
After that, it's just a case of how much can they live off that period? This is why artists like Fleetwood Mac and Paul Simon (sorry Roger) have achieved the almost impossible, and had a rebirth. Fleetwood Mac with Rumours, and Simon with Graceland. Before that, they had become footnotes in musical history.
The Stones' last great album was Sticky Fingers, Bowie's was Ziggy, Elton's was Yellow Brick Road, and so on. I reckon that the great writers have about thirty great songs in them, and they are all spent after a couple of albums. Elton John did the clever thing and sprinkled his good songs over four or five albums. He hasn't recorded anything decent in twenty years.
McCartney has become a travesty. Both he and Lennon fed off each other, vampire-like, during the Beatles, and the residue was still there after they split. McCartney did one great album, Band On The Run, and so did Lennon, (Imagine) and that was it. In terms of creating good material post-Beatles, Harrison outdid them both.
The brilliant Alanis Morissette managed to record one of the best albums ever with Jagged Little Pill and I can assure you she'll never record another great album. I believe musicians gather all their material through their formative years and it's all gone in the first five to ten years. Morissette has nothing left in the tank.
I've done the clever thing and left my recording career until my fifties, so I still have a wealth of material ready to lay down, I reckon two albums worth, but unfortunately I'm now too old and unattractive to get a contract.
Unless I re-invent myself as a country singer, that is, but even then I'll need a stupid but memorable name. Just call me Hank McEnema.