2 things I've noticed are behind this question: 1. In the Giro today (no spoilers here) Paul was talking about how the avg.speed might break the record which has held since 1983. Now I'm probably more familiar with state of the art from 83 than I am today. A TdF bike then weighed 20.5-21.5 pounds a good 4 pounds more than current.
#2 is the every now and then thread about someone showing up for a ride on an, let's say, "inappropriate" bike but still keeping up; apparently to the amazement of all.
Obviously the engine is of supreme importance and human beings haven't changed in 20 years so how big of an advantage is the bike? Noone doubts that a time-travelling Eddy Merckx could compete in today's peloton but could he do it on that Molteni Orange steel friction-shifted antique of his?
Maybe I've got too much time on my hands if I worry about things like this.