I think most of them die from being wrecked or sitting. Either because the owner got scared of it or due to neglect.
I would guess very very very few of them get "mileage dead"
If you do actually manage to wear a 250 engine out you either do A LOT of riding like the 80,000 mile engine guy. Or your maintenance sucks. I.e. way too long between oil changes etc etc.
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Viper: Good morning, gentlemen, the temperature is 110 degrees.
Wolfman: Holy shit, it's Viper!
Goose: Viper's up here, great... oh shit...
Maverick: Great, he's probably saying, "Holy shit, it's Maverick and Goose."
Goose: Yeah, I'm sure he's saying that.