I looked at some numbers before and after Nadal took #1 from Federer after 2008 Wimbledon. It is widely accepted that Federer was out of his peak by that point when he had just turned 27. This is actually the average age of Nadal and Djokovic right now. Doesn't look like they're slowing down due to age anytime soon, yet one could argue their style of play is much more punishing on their bodies than Federer's.
Either way, let's look at overall win pct. and GS win pct. before and after 2008 Wimbledon for Federer, Nadal, and Djokovic.
Overall Win Pct.
Before and including 2008 Wimbledon:
Nadal - 309-73 (80.9%)
Federer - 594-143 (80.6%)
Djokovic - 159-60 (73%)
After:
Nadal - 349-56 (86%)
Djokovic - 384-72 (84%)
Federer - 329-72 (82%)
GS Win Pct.
Before and including 2008 Wimbledon:
Federer - 155-25 (86%)
Nadal - 75-13 (85%)
Djokovic - 46-14 (77%)
After:
Nadal - 96-10 (91%)
Djokovic - 112-16 (88%)
Federer - 105-16 (87%)
What stands out to me is that Federer's overall win pct. and GS win pct. is actually better after 2008 Wimbledon than before (this includes his down year in 2013). There is clear improvement for Nadal after and Djokovic was obviously at a much higher level after.
The place where you see the dropoff for Federer is in the H2H against the players that he has met the most times (the top 4 for him are Nadal, Djokovic, Hewitt, and Roddick). Looking at those 4 his H2H before and including 2008 Wimbledon is 41-23 (64%) and after it is 24-25 (49%).
It looks like Federer's results against the majority of the field have actually improved after the age of 27. Where he's done worse is against the players that he faces the most, mainly Nadal and Djokovic, who have clearly improved after that point. Does this mean his overall level dropped, or a few players improved and surpassed him?
So, Fed's results vs the field (without top 4) are about the same in 2011/2012 as were in 2005/06.
You've actually proven a weaker era. That means no young guys to challenge Fed and amazing talents from Fed's era declined too.
Because we know 100% that Fed's game declined in 2011/12 compared to 2006. If Fed is being worse and having the same results, that means outside of top 4, the field is weaker.
We have many evidence where Fed declined:
-Hystorical evidence - Every all-time great declined at that age
-Fed's numbers vs top 4
-Naked eye - Everyone can see he is slower and less explosive
-He makes less winners and more unforced error
-Medical evidence - says any player declined when 29/30 with that kind of mileage
-Basic laws of nature - everything decays and slows down.
So, declining Fed has the same numbers vs the field (bellow top 4).Your numbers are great proof, that the field bellow top 4 got weaker.
Young guns are not as great as they were in Fed's era, where there was Murray, Berdych, Tsonga, Nadal, Djokovic, Del Potro... Compared to Dimitrov era.
And Fed's era had much more champions than today. Today we only have top 3 champions: Nole, Rafa, Murray. Fed's era had: old Agassi, Roddick, Hewitt, Safin, Ferrero, older Kuerten and great guys like Davy, Nalby, Gonzo...
So, no wonder declining Fed can maintain his numbers vs guys outside of top 4, since the field is not as deep.
I would like to see for Sampras for example, how he did vs guys outside of top 4 after his decline.