most of the "dead" one I know of had physically broken cranks (as in the picture I posted yesterday) which was usually due to vibration, the general theory behind the design being that the crank lasted a literal lifetime.
you are right though that a merely worn or exposed to the elements crank is not beyond repair, technically at least.
in reality I can remember a scrapyard in cornwall simply smashing what was clearly well over a dozen CS singles and twins that had nothing more wrong with them than being shut down one day and left standing idle for years, because it compacted the scrap.
we did the same thing (breaker bars and gas axe) to literally tens of thousands of tons of perfectly good steam locomotives 40 years ago when the rails were electrified and diesel powered.
the best source now for listers and the like is farm auctions, but you have to be in the uk and able to travel with chequebook and trailer and be in the know to hear about them (for instance there's pughs, but little point me giving links to that sort of stuff here) so you have twenty lots that go for good money and 200 lots that go for basically scrap.... I could buy running or damn near running farm tractors like fergie T20's for 200 pounds a pop and fill a hangar with them, but they are worth more as scrap and that's what is happening to vast amounts of stuff every day.