Not sure what/if figures are available but the general principle is well understood: new sponsors with deeper pockets arrived in the sport (the people behind Moser's Hour record, Tapie etc); Guimard and Hinault broke the power of the criterium agents and riders started to get paid proper and not have to rely on crits/Winter Sixes; the points system focused efforts; as a result of these changes teams became more and more professional (we're still seeing just where that can go).
Whereas before sprinters like Van Looy and Maertens and Kelly could imagine winning Grand Tours and GT riders could win the big Classics teams stopped gambling like that and specialists took over. Without going too far into forbidden territories EPO was in part a product of those changes and itself changed the sport, favouring GT riders more than it did one-day specialists (in-race recovery, not just recovery from training).
None of this is meant to take away from the current generation of talented riders but the wider context matters when people say 'we haven't seen the likes of this since Merckx'. First, we have, and second what happened in the nineties and noughties has an explanation.