Do the cups of headsets with cartridge bearings wear out?
The bearing races themselves get replaced, but what can happen is the races becoming loose in the frame. I can't imagine that happening just through wear, though, you'd have to pull and replace the bearing races hundreds of times. Normally that is the result of either a crash deforming the races, or the bearing not being quite square when someone hammers it into the frame. To avoid the latter use a proper bearing press or fake it with a length of threaded rod. In general, any time you find yourself wanting to use a hammer on a bike you're doing something wrong.