Players like Brazilian Ronaldo, Shevechenko, eto, nistelrooy and especially raul played with amazing players too. I wouldnt say XAvi and iniesta are better than zidane and ronaldinho and deco. If anything the manunited Nistelrooy was in arguably had a better squad than Ronaldo's. In other words, my point is that the older generation had star-studded squads as well.
Moreover, If you look at Pep's barca and current madrid you'll see that Messi and ronaldo are the only ones that have crazy numbers of goals and you can always see how the team's goal scoring ability is diminished in their absence. Players like Cavani, Ibra, Aguero, Suarez, Benzema would be world class players in any generation but still none of them can come close to the goal scoring feats of Messi and Ronaldo. Today we have star studded teams like PSG, City, Chelsea and yet their top scorers aren't able to reach the effectiveness of Ronaldo and Messi.
I do not doubt that the sports and medical sciences have advanced a lot over the last decade and that the ball, modern pitches and contemporary attacking strategies favor current strikers over the previous generations however these advances affect all players at top clubs today and yet only two players seem to be on a level of their own far away from all the other super stars. They arent just pure goal scorers either, they also assist a lot (messi especially) and have a very wide range of skills. Whichever way you look at it, Messi and Ronaldo are anomalies.