Sure, I agree with the bolded part. I also think if Fed was closer to their ages, they would have shut him out of a lot more Slams....but I digress
The point is that in 2004 you had one young pup proving himself to be indisputably the top dog against a bunch of non-ATGs. And you had one fierce old lion (and ATG, Agassi) trying to put Federer in his place. It was exciting, you could tell you were witnessing history. It was a legendary season, no doubt. However...
In 2019, you have three old lions (and THREE ATGs) fighting for the Slam race. And none of the shoulda, woulda, couldas matter when you look at their trophy cases. A 20-16 match will always be more impressive than a 4-8 match, because one match could decide the max number of Slams ever won.
No, Federer was not at his best, by any means, but he still had a lot of fight in him. And Nadal and Djokovic were still quite impressive.