I think you're mixing apples and oranges here (grandmas and frogs (babe i zabe) we would say in Serbia).
I agree that comparing athletes from different eras is very dubious, but it can be done if you define a criteria. And that's the problem, how to define an accurate criteria when sport changed. It changed in goals which riders target, in the way they specialize themselves (these two are regarding the number of races won criteria).
It changed in equipment, training, supplementation, logistics...
And regarding this second, technical, criteria, I want to point out that average speed difference indicator can only be attributed to technical criteria. There's no reason to believe that in just 20 -30 years (that's just one generation) superior people are born. Just look at averages of Indurain and Armstrong era, when conditions are similar - differences are smaller, nearly invisible.
So, my standpoint is - Coppi, Merckx, Hinault, etc. would be equally good to modern riders. Moreover, I think they would share domination with today's dominators in GTs and would be better than today's riders in classics (you can notice here, on classics example, how specialization came in sport - further you look in past more complete riders you see, closer you look - more specialized). They'd also share each other's domination (in their eras) if they raced against each other - Merckx wouldn't have won so much if he had to race against Hinault, Armstrong, Coppi etc.
I'd like to hear what do you think achievements of modern riders would be if they raced in conditions when, for example, Coppi raced. I didn't choose Coppi by chance. I believe that modern riders wouldn't have big problems accommodating on conditions in Merckx and Hinault era, but conditions in Coppi's era were something else.
But then again, you accommodate on your surroundings not even thinking how it could be, or if it could be different, it is like it is.
It would be very funny to watch modern riders on bikes from Coppi's time, on unsurfaced roads without domestiques, radios... They don't want to race if radios are banned just for one stage! And that's the aspect of racing which would least harm them. Try to put them on unsurfaced roads or ban them from taking water from team mates...
On the other hand, Coppi would enjoy riding in 2010... It would be like cyclo tour for him.
But, just like I said - it is like it is.