Alternators can go bad for any number of reasons, A short in the stator or field coil, an open or shorted diode.Or a regulator that is stuck full on, or not at all. Bearing failure is another .
Most alternators die of old age, when the brushes get so short, they can no longer contact the slip rings. Or the housing gets so full of crud it can no cool the stator or diode heatsink.
When I worked on buses, they had 24volt direct drive, brushless, oil cooled alternators. Most of these were changed from the old engine to the new bullet at around the 500,000 mile range. I never knew how long the alternator was run. No need to track it, as failures were uncommon.