A 125/150 is good for at least 50,000 miles if properly maintained
14 lifetimes ago, take my recollection with some skepticism, it was the '60s, a friend of mine had a Sears 106 with over 100k, his dad was a small engine mechanic at the local Sears service center. The original owner brought it into the service center for every scheduled maintenance.
When my friend bought the 106, he sold me his Honda Super 90 that every other kid in town had already owned, my first bike, a turd. Maybe if bikes had rev limiters back then, it would have survived longer.