Ridiculous. Kuerten is simply not as good as Nadal and Djokovic, not by a long shot. They move better and they hit the ball better and are more complete players period.
And you know what? Murray regularly gets his clock cleaned on clay even though he's a great player and fast as hell.
So? What does that prove?
I'll tell you: it says something about those guys specifically and NOTHING about your supposed argument.
In any case it is very much about fitness and training as well. A guy who has great movement has at least some things to fall back on and doesn't get knocked out so easily on a not so great day though he can still lose of course.
You're absolutely insane if you think that Murray just has to play his game on clay and he'll be in the RG semis and finals or that Nadal can play at Wimbledon the way he played at RG and he'll breeze through.
In any case of course you'll believe what you want to believe.
He'd do even worse if you just changed the ranking system. If it were best-fourteen and no mandatory Masters events, Ferrer and sundry others would probably focus on clay full-time & there would be more difference in results based on that. More players with the potential to fluke Murray in the first two rounds. Murray OTOH probably wouldn't even show up to Paris most years.
Thomas Muster's best results off-clay when he reached #1: win at Essen Masters (carpet), R4 at USO, R3 at AO, and a QF at Indian Wells. His rankings also had something like 12 wins on clay. Nobody has any incentive to take on a Muster-like schedule anymore; if they did, the FO would be a lot trickier.
The surfaces aren't necessarily any different--they are, but they aren't. It seems like they've been toned down to the same extent that the racquets have been tweaked up. What this means is that the ball gets from point-of-contact to surface a lot quicker, and then from surface to racquet more slowly. The ball gets from point A to point C in the same time as normal. But the flight is faster and the bounce is slower, so there is almost never incentive to crash the net without a decisive winning advantage in the point. So every court plays the same in that respect: stay at the baseline or else you will get burned by the modern-day super-raquets.
IMO they can do what they want to the court, I'll watch either way.