"Could riding style be a factor?"
On the old mountain bikes, no question it was a matter of beating the heck out of the thing. Cassettes were not a thing quite yet. On the touring bike, the bent axles came from 40 pounds of gear and 200 lbs of rider all on the back wheel. No big deal to replace, however should the axle fail completely out in the middle of the pan handle of TX, that will be a problem.
Having a thing for old bike stuff is becoming less practical every day I think about this, and I don't want to give it up! Looks like the smart thing to do is go cassette.