maddog's experience mimics mine. I saw a lot of BMX and messenger service work back in the day and we rarely saw really busted up bearings. But back then the balls were larger (in so many ways...), remember the load capacity of a ball increases in a geometric way WRT diameter and only linearly WRT ball number. Do the math and you'll understand why the "classic" eras had bigger balls in their bearings. Then break apart those cartridge bearings so many swear by these days and see how tiny in diameter the balls and how few they are.

To the OP- I suggest figuring out the chain tension when the chain has some loose points and the chain is tightened at its loosest point really tight and then pedaled past that now super taught point. I suspect you would be surprised at how much tension can be produced by poor set up alone. And remember this tension is all the time, even when no riding is going on. I could see fretting being an added path to bearing degradation.

I try to be a student of history when it comes to bike problems. When a type of "failure" (how I dislike that term...) happens over and over and is reported by many in other places it seems to make sense that a shared experience and understanding might result. Andy