They were better back then. There may have been dope back them, but I think was probably less effective than the dope of today, which was/is widespread, so if anything, a doped yesterdays man was probably at a disadvantage to a doped modern rider.
Either way, to ride with a heavier biked, less effective training regime, and something as simple yet limiting as downtube shifters, and to win like Merckx did, that was something. As hungry as Lance is, I know who I'd put my money on.
Remember, a lot of those guys came from a hard background - peasant stock, farmers grown up on the land or manual workers, grown up in the ruins of post war Europe. They knew real hardship. Real hunger. That puts something in you that a training regime and good diet never can.