My thought is that riding past the peak of the power band causes excessive valve seat wear. Or possibly excessive and unnecessary oil changes. That would be an interesting thought to investigate.
Another thought is high start up wear as a result of near total drain down on bikes which aren't used every day.
As the return of the valve is controlled by the springs with no cam interface at impact time, seat impact and any possible slight stretch is not affected by the revs. Higher load forces on the cam silencing ramps at high revs would lead to cam or shim face wear which would open up the clearance rather than close it. Interesting.
Intuitively you expect harder use to lead to higher wear, but I can't see the mechanism for that on the valves. On the bottom end you can see where greater loading on the friction surfaces and bearings increases wear, but on the valve train there's only higher revs, not increased loading, except a slight increase at the lobe/shim interface as a result of the higher revs.
My only experience of Kawasaki as a long term owner was the ER-6 so Kawasakis might wear more than some other brands. Don't flame me for that - it's just a thought. The ER-6 had all the valves in spec when I sold it at about 25K miles. Given the mileage I gave it a full service and put it through an M-o-T for the new owner.
This is going off thread, but it's got me interested.