I wish someone could testify to this. You would think after what, 5-6 years, watching these boards(multiple forums) we(you and I, Leo) would see someone saying "Mine has been working well for XXX miles well past the 15-20K mark".
Here's some honest truth. When I bought ol' Trigger she had 17,000 on the clock. She now has 49,000 and therefore I can state unequivocally the cam chain and guides have lasted 32,000 miles. Since there's no sign that the engine has ever been opened up, you might speculate and say the chain and guides have lasted 49,000 miles so far. I've tried to take good care of the motor and there were signs that prior to 17,000 someone else was doing that too.
The only problem is that suddenly she has started to use about maybe 1qt of oil about every 500mi. At highway rpms. The bolted on plated under the left cover worked loose and leaked some oil and I rode it like that for about 500mi at 100^F temperatures. Check your oil every time you gas up!
But she has also started to trickle oil from the head gasket, so I'm hoping a re-torque will fix her oil issue. That will probably have to wait till spring.
I would really hesitate to replace genuine yet old Yamaha innards with new Chinese copies at a certain mileage just as a matter of course.