I simply replied Merckx wouldnt beat Vingegaard in the mountains or Van der Poel in flandern or most of the peloton today even.
really hard to say.
even Lemond says that had he known that if he lost 5 more kgs (even from his TDF weight) he would have registered much higher watts/kg -- like they do now.
also, while Merckx had a great team, the team leaders (and most often Merckx) would literally ride most of any stage in the wind -- leading the front group. There was not this tradition of the mountain train at all.
then there are the technological advances....
lastly, the design of the routes DOES matter. Having 30 KMs of ITT or whatever in a GT literally alters who can challenge and, more importantly, how they go about it. Look at Remco desperately losing 2.5 KGs in 3 weeks. Look at the skeletons and the eating disorders -- this is all connected to trying to fulfill the audience demand for nothing but mountain climbs. You have 100 KMs of ITT. You have a TTT. You have many 200+ KM flat stages (that wear out the skeleton climbers) and you have a completely different competition.
merckx was a freak of nature to be sure. however, it is likely that Lemond was even more so. and comparing with today is pointless.