Dear Dances, please let's not start a religious war here. Even though development of a hydraulic tappet is not on the table right now, it doesn't mean it can't be a topic in the future.
"If" is not a hypothetical statement, rather it idicates an uncertainty as to what is really going on at the lobe/tappet interface, which no one really comprehends according to my understanding of what has been written here. If you know otherwise, you should prescribe a solution right now! "Proper cam and valve design" is a rather vague and general statement! Sure - we want a proper design, but how? Is the valve train of our Norton engines "proper" otherwise? Obviously not, considering the camshaft flexing, etc. How well-designed the rockers, valves and springs are I don't know.
Apart from parts failing due to obvious manufacturing defects, which doesn't need further consideration, there are basically two known factors occuring at the camshaft lobe / lifter interface: Abrasive wear, and valve train dynamics (the hammering effect) causing spiky contact forces past the camshaft lobe apex. I haven't read a statment by JimC in which he correlates hammering and valve lash. This would be an extremely difficult observation, with the lube oil present, compression of push rods, deflection of the camshaft, various thermal elongations, etc. all occuring simultaneously. Maybe a high speed non-contact laser sensor paired with a built-in compression sension in the lifter enables recording time series of the lobe/follower contact area which would allow such statements to be made. The best laser sensors provide a resolution of .04 micrometer only and on short range will probably penetrate the oil splash. However, I don't think JimC is equipped for such measurements now.
It's interesting to note that abnormal wear of the tappets also affects the BMW airhead engines, despite that maker's rigorous quality control.
I rest my case.
-Knut