Well, my R25/2 might not be the "north star". I put only 300-500 miles a year on it. So 20 years might be 10000 miles? Hardly a challenge for the coating.
My recollection was that with the '81-on models which come equipped with a Nikasil coating, the thought was that one would run 100K miles, rering, and go another 100K miles. Snowbum has a lot on Nikasil here:
https://bmwmotorcycletech.info/cylinders.htm