Re: When there's no more magic left it's time to stop.
To me, if that's the case then he sold his discworld legacy cheaply to make a political point, although his books had been increasingly formulaic (disaster looms, hero/ine has some special ability, they or nearby character falls in love, tension peaks, everything is OK in the end). Not that I didn't enjoy them, but as western tech was increasingly used to prop up stories so they became decreasingly interesting.
I read quite a bit of his output, including the truckers series, dark side of the sun and the carpet people. Each had their own flavour and each bore his style. Perhaps discworld became his vehicle for protest because he thought he'd affect more people that way, and was too far gone to create a new world instead.