I've always thought the "you can't ride/win xxx on mineral water" excuse was nonsense. Yes it's the riders that makes a race hard and if nobody doped then you can actually ride it on mineral water.
To put the "you can't ride/win on mineral water" comment into some form of context, when
Jacques Anquetil said that only a fool would imagine it was possible to ride
Bordeaux–Paris on just water*, the kind of knowledge about training, nutrition and post-race recovery available now just wasn't there, and with a racing schedule heavier than what most grand tour GC contenders put themselves through today, riders ended up reliant on the amphetamines that fooled their body and mind into carrying on. That's
not to say it was right, but the riders themselves didn't really know any differently.
Prior to the advent of steroids, growth hormones and oxygen vector doping, brandy and Benzedrine weren't enhancing the riders' performances in the same way, it was more that they numbed the pain and fooled the body and mind into pushing on beyond fatigue limits. It's pure speculation, but imagine if a rider of Anquetil's generation had access to current knowledge on training and nutrition - he probably could do just as well without needing to pop the speed tablets.
* That probably goes double if you've just finished riding the Critérium du Dauphiné beforehand.
http://en.wikipedia.org/wiki/Jacques_Anquetil#Dauphin.C3.A9_and_Bordeaux.E2.80.93Paris_double