I wouldn't limit them to 40k. I've seen quite a few get to 60 and 70k, but if you take excellent care of them they can last about 80k before they can't be adjusted anymore. Don't remember what part actually wears out, but it doesn't justify replacing it and not rebuilding the engine.
Bikes are able to run way past that, but most of it depends on whether they are maintained or not. If you put money and time into your bike to take care of it I have no doubt it can ride you to 80000 +
If you look at people in Europe that ride the motorcycle as their daily everything - you will see some outrageous mile numbers.... Of course everything is dependent on the mechanic (yourself or other paid person)
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