I am still smoking a few MMs I bought back in the 1970s. One in particular...the Bulldog, discontinued for quite some time, is till one of my favorite smokers. I bought it sometime in the 1980s. My oldest is a Legend, bought in 1972 for around $3.00 (I think). It was one of my first pipes, and still smokes great.
I've never had a burn-through. Other than just out and out breaking or losing them, all I have ever done is burn the rims, by over-filling them. You can fix that easily just by sanding the bowl down past the burned area.