I also agree this thing looks like long term abuse and/or damage. But it really hadn't had any. And I never saw anything on the drain plug's magnet and only the finest and fewest filings in the fluid.
Wasn't suggesting that you abused it, but I'm sure you already know I wasn't.
My next best guess is just a poor casting of the gears that eventually did what they did, and having quality gear oil in the case kept it together longer.
Just think, if you had the Syn 3 junk we probably would have been making arrangements to get the bike back from York!
18+ years as a "A" tech specializing in engines - transmissions (manual and auto)- and rear axles I have seen a lot of failures, many just like yours and quite a few with absolutely no plausible explanation. Chit happens.