They do break, not so often but they do break.
I've seen some in the last 3 months from beginning '90s and all had a crack at the seattube/bottombracket.
Also the full sussers crack. The frames I saw did not realy break in pieces but just cracked and made noises...always a bummer if you have to replace a golden oldie for a Trek...
I know some people with old Mantras that have been well ridden and still intact. My old Adept and current Palomino have been fine. (Apart from the build quality on the Palomino)