From a purely physical respect... I honestly don't think someone like even the great Merckx in his prime would win a Tour de France today, much less five. And if he couldn't, Anquetil (sp?) couldn't, nor could Hinault. Merckx averaged 38 km/h in 71. His other wins were at an average of 34 and 35 km/h. (Yes, hard to compare because of course particulars and weather). Hinault won two years at the same 38 km/h speed. LeMonds were 37 and 38. From 1991 on, the average speed of the winner has gone from peaking at 37 or 38 to being the norm, to being on the slower side. 2007 was the first time since the 1994 it's dipped below 39 km/h. Generally, they're at 40 or 41 km/h now.
Some of this is equipment, perhaps, but increased popularity and improved training methods have created stronger riders. My feeling is that outside of Merckx, Hinault, and LeMond, no other winners would be able to hang very long with todays riders and even they would be suffering, none to finish on the podium.