It's not surprising that alternators last as long as they do as the only wear items are two bearings and a couple of slip rings in the voltage regulator. Since the voltage regulator is a service item, replaced with two screws, an alternator might easily be expected to last 500k miles or more.

The fewer moving parts and wear items, the longer the expected life. There are gas turbine engines that run continuously in oil and gas applications that have a TBO of up to 100k hours or 11 years. Hypothetically, if they were powering a car running at 50 mph, that would equate to 5 million miles. I'd venture a guess that the overhaul requirement is based on the turbine hot section, not the bearings.

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