I've never had a sprocket move on my Norton crank, but I have broken a key under a Triumph clutch. The amount of purchase is different The engine sprocket turns about 3 times as fast as the clutch, so the load on the key is one third.
Not the best Idea engineering wise but some get the clutch center and shaft broached for a second keywayDitto - all the 'lapping in' repairs on a previously spun Triumph clutch have been a big waste of time and effort for me: The only effective fix wass to replace the mainshaft and the hub centre.
Interesting point regarding provision of a weak link via the taper & key. However well-intentioned it was I've cursed Triumph for it many times!
Never heard of a triple mainshaft spinning, but lots on the twins
I can't remember if the triplex Commando sprocket has threaded holes for a puller.
Bottom line is a taper shaft will hold providing it is the correct type for the job and has clean mating surfaces and is held on securely by a nut, or bolt that itself has a locking washer-but not always.
Even the boat propellers on big ocean liners and freight ships employ them.
https://sciencing.com/calculate-shaft-taper-7564452.html
FYI the works Norton John Player bikes went to duplex primary chains. . . . .I've got three single row engine sprockets off Nortons. Only one of them gives the correct chain alignment on a Commando with a Manx clutch. How many Norton models have triplex primary chains ? If you are racing and your gearing is not right, you usually go backwards. With a Commando, you don't know how much torque your engine has, until you raise the gearing and the bike accelerates faster. They won't spin-up fast on the throttle.