It is basically impossible to compare eras because of the reasons you mentioned. There wouldn't be a Frazier without Dempsey and there wouldn't be a Tyson without Frazier and Dempsey. Every generation was built on the previous one and added skills/nutrition to the evolution of the sport.
It is interesting that recently I read on an MMA forum that Royce Gracie is not an ATG and that he is basically a nobody. That is coming from people who don't know history. Gracie was the first guy to introduce BJJ into the UFC. Basically Royce BUILT the UFC in a bareknuckle era where there were (almost) no rules and he was fighting MUCH BIGGER guys.
So some bum coming along 20 years later saying Royce is a joke is kinda.....funny.