Chanwan
G.O.A.T.
I agree that worse results after 2007 would be due to both rivals improving and Fed declining, but I think the curve for improving is much sharper than the curve for declining. I don't think you can be as dominant as Federer was from 2004-2007 and just lose it suddenly the next year.
As for the performance against top ten players in 2008, as you said he had mono. But like I said, the GS performances to me show that he could pull out high-level tennis when he wanted. I would say some of the losses in the smaller tournament especially early in the year were due to him not wanting to push himself when he's not feeling 100%. That does not indicate a decline in his level due to age, for me.
Later in the year, you have the loss to Blake at the olympics. Which again, Federer had never loss to him, but I don't see how you can expect Federer to never ever lose to a quality player. I've seen the match, Blake was apparently extra motivated to be representing his country at the olympics and played at a very high level. He had a couple losses to Simon, but this was his best year, finishing the top 8, and why Simon was even at the WTFs to play Federer. They had never played before, so it's not like Fed had dominated him before 2008, and the match at the AO 2011 showed that Simon's game could cause problems for Fed when he was completely healthy. Murray clearly was improving by 2008, made the USO final his first GS final, and had beaten Federer before.
So, overall I don't see a sudden decline in Fed's level in 2008, I see some other circumstances.
As for the first bolded part, I don't think you're completely right.
Think McEnroe in 85 (and the rest of his career), think Wilander in 89 (and the rest of his career). Think Sampras' big drop in 01. There are losts of such instances, each having their specific reasons.
As for this particular instance, I think what partly caused it was the effect of mono. Sure, other players improved too. As for whether it was age related or not, I'm not saying it was primarily age-related. Rather, I'm saying that he was young enough to stay on top of his rivals regardless of his age. But that an outside factor - mono - interrupted his 'prime' or whatever we shall call it and made him weaker than he otherwise would have been at that age.
In effect, I'm saying that had Federer not had mono, I would have expected his '2008 year' to have happened a year or two later.
And yes, he could 'push' himself and get extra motivated for the slams. And there probably was a whole lot of fear factor acting in his favor as well. But that still doesn't mean that he played at the same level in the slams - just that his best was still better than 98 % of the other players.