I just read the long term test of the K75, dating back to 1986. After 42k km (26k mi) of pretty hard riding (like flat out on das Autobahn for hours on end), the bike was completely dismantled. Inside, most of the engine was like new, apart from one mysterious fault; the fourth main bearing on the crank was absolutely shot. No other damage was found, the oil filter was clean, and the crank itself could be reused after lightly polishing the crank journal. A freak occurrence that nobody could explain. The only other issues was a clutch out of line with the output shaft by 0.22 mm, causing the splines to strip after about half the total test distance, a fork that sprung a leak at the seals, and a shock that wore out and lost most of its damping.
I also have a 100k km / 62k mi long term test of the K100RT that I plan to read someday.