When we split the final drive off the swingarm, it actually dripped rear end oil out of the joint a little.
This might be a piece of the puzzle relative to why some dry out and why some don't.
The oil seals (yep, that's what that donut shaped seal is called in the manual) may be failing.
The manual calls for them to be changed at every service, but I don't know of anyone who does that. But, our bikes are getting old enough that it might be time to start changing them out.
I reinforce the seal by packing its groove with grease before pushing it into the cup, (The manual implies this step with an icon for a grease gun pointing at the seal, but it could be intending to point to the splines and just be badly drawn.)
I then smear what oozes out around both the interface with the pinion cup around the outer circumference, and around the interface with the drive shaft. I like this extra insurance. I think it helps the seal do its job, keeping lube in the pinion and any water that happens to enter the swingarm out of the pinion.
75k on one and about 60k on another and so far, so good.