This is a very interesting talk by David Epstein
, author of The Sports Gene, a book which I have discussed here before. He points out that Eddy Merckx’s hour record was extended only 10 m by Boardman, when he was restricted to a similar bike. Sosenka later added about 250 m to that, but he also tested positive several years before and after that record. Merckx did set his record at over 2000 meters altitude, which would provide a significant aerodynamic advantage, though some of that would be lost because of the reduced oxygen levels. I think the most favorable estimates are that he might have gained 2-3% in speed, though most subsequent attempts have been at sea level, by choice of the riders.
Epstein also claims that if Jesse Owens ran on the same kind of track that sprinters do today, with the same kind of shoes, he would only be a couple of steps behind Usain Bolt, instead of the 5-6 meters I guess it is that his actual time implies.
The point being, then, that the physiological improvement of the best athletes over long periods of time may not be that great. If no one today can ride an ITT faster or much faster than Merckx, using the same equipment, it strongly suggests that no one can climb any faster than him or other great climbers, if the equipment, roads, and other non-physiological variables are the same. TT success, of course, is power/surface area, while climbing is power/weight, but unless one believes that new, allowed methods enable riders to lose weight while maintaining power, we would expect any discipline involving power to show the same lack of improvement.
I still think the larger pool of riders could have some effect, that is, the odds of another Merckx or even someone superior to him increase with a larger pool. But other than that, Epstein’s work suggests that if riders today are climbing that much faster than Merckx, Hinault, and some others of the past, most of that difference has to be due either to changes in equipment and possibly roads, or doping--as opposed to training and nutrition. I suppose one could argue that not many riders are that interested in the hour record, particularly the one using the more traditional bike, and don't focus on it. But surely if the differences we see in climbing times reflected differences in physiology, many riders today would have a very good shot at breaking the hour record, and might want to go for it.