The argument that Fedor (or anybody) can't be the GOAT because current P4P bests are better than he is when he was in his prime are both nonsensical, and accurate.
It's nonsensical because EVERY new generation will be better than the last. The new guard beats the old guard, new fighters improve on skills and evolve beyond the old fighters. If your definition of GOAT means "the best current fighter in the world", then there will be a new GOAT every 5-6 years or whatever. If we are going to decide GOAT, then you decide it on their era, the fighters in their era, how impressive they were in their era, etc. Fedor compares very, VERY favorably to Anderson/GSP/Jones if you consider the era and how impressive he fought.
On the flip side, it is accurate to say that Fedor isn't the GOAT, because there WILL be someone better than he is (arguably there already is), and there WILL be someone better than Anderson (there already is) and GSP and some day, Jones too, and NONE of them will be the GOAT, on a technical level, as there will be new fighters, with more and better skills, who are technically superior.
The argument of "GOAT" is pretty useless all around, too many variables and on a technical level, the GOAT is the current best fighter in the world at the time of the debate, so that would be who? Jones? Maybe Robbie? Conor if he beats RDA? Then what happens in 5 years when someone has knocked Jones out, defended his title a few times? He's now the new GOAT, right? He's better than Jones fighting tougher competition than Jones did, right? It's just an unimportant argument that goes nowhere.