What you’re saying makes little sense and is just another way of devolving to the default that eras are just too different to compare and therefore are all the same -- which is ridiculous and oxymoronic. In your mind it seems that the only way to think fairly is too assume that Federer faced competition that was just as strong in 2004-07 as it is now or ever; which again is ridiculous. Also, you bringing up the 90's is a stretch. It is certainly easier to compare 2004-07 with 2008 to present, than either of these time periods with the 90's. With the former we have constants or relative constants: mainly Federer himself, the homogenization of surfaces, and the introduction of the new poly strings. With the latter comparison (the 90's vs Federer's time) we have none of these constants which makes the comparison much more difficult, yet not impossible.