How many slams has Federer won the past 5 years? His first 5 year slam main competition was against: Hewitt, mid thirties Agassi, Nalbandian, Roddick, Baghdadis, Safin and even lesser players. The came Nadal, who could not win a slam off clay till 09, then Novak who did not reach his peak till 2011. Pete had Agassi, Becker, Edberg, Wilander, Chang, and Courier among others to contend with. The Connors-Borg- Lendl McEnore era was also much tougher than Roger's 03-08 as well as the 09-13 period.
Err...In the past 5 years, a declining Federer has won 5 slams, which is only 1 less than the current world number 1 has in total.
And while the death of fast surfaces started under the nose, it's peaked since then. The average age of a professional tennis player is 26.5, Federer is 31
, and regardless of what you think he was in his prime, he is clearly not the same player he was then. The last time Federer was anything close to a complete player was 2009 - and he's still won 2 slams since then! There has been 5 slam winners in the past 100 to win over the age of 30, and they were before the game became the athletic grindfest it is today.
And speaking of past their prime, Hewitt still looks better than most of the tour despite not being anywhere near his prime in years. Even Nalbandian had moments of brilliance in the past few years despite being over the hill and out of shape.
Also, Baghdatis is closer to age to Nadal than the generation you are talking about. He peaked early. If you are going to play that game, you can mention a player from Federer's generation who was a nobody until recently - Ferrer.
I don't consider Federer anything close to the GOAT and I'd take the 90's and both early and late 80's over the mid 2000's, but Federer, Safin, Hewitt, Roddick, Ferrero and Nalbandian only wish they were younger, they'd clean up nowadays. Hell, second rate players like Gonzalez and Davydenko probably could have won a slam had they been a little younger.
The last 5 years have been the nadir of tennis history.