Starting with the more common problem of a broken strut... A
strut can (very occasionally) break on any make or model of boat with a shaft drive. Actually the more common problem is electrolysis which can occur any time a boat is in a marina with improper wiring or just parked too near another boat with improper internal wiring. This would cause several days of work at yard rates.
A friend of ours always kept a lump of zinc hanging over the stern of his sailboat, connected to his boat's ground system, at his slip in Pt Angeles. It was/is common for huge motor yachts to tie up within 100' of his head walk and some of them were leaking current into the water at an alarming rate. (And his sacrificial zinc "fish" would start diminishing immediately!)

He said that sometimes complaints to the port captain who then had to confront the captain/owner more than once were required.
As for the
shaft and coupler, it's common for one of those parts or both to need replacement after decades of use. This will depend greatly on how well the old packing was maintained (such that the shaft was not scored) and whether leaks or seeps had allowed sea water to corrode the coupler. Even small amounts of salt water, over time, will corrode the shaft and coupler to the point where it's really difficult to separate them. We changed over to a split coupler design over a decade ago, just to reduce this risk. Even if corrosion had "welded" the two parts into one, the problem would probably be resolved by a mechanic armed with a saber saw in a few hours, followed by replacement with a new trued-up shaft.
BTW, I have to wonder where the reference to $50K for working on the stern gear comes from? That sounds about 10X too much to me. Wow.