One thing I don't get about FEderer is why did his decline happen so young? It happened around middle of 2007, imo, and then he was 25, nearly 26. That's very young to have declined to be honest. Look at Djokovic, he's gonna be 26 and has been playing for a pretty long time now and it doesn't seem he has declined, same for Murray. In fact it seems both have improved in recent years rather than decline. Nadal has also declined but it wasn't a surprise given his knees and his playing style. But even then NAdal's level, before his injury at least, was very high. I think Federer declined 2 years too early.
The only proper answer imo is it didn't happen in 2007 at all, it started only in 2010. What happened in 2007-2010? The competition improved, and I think Novak, Delpo and Murray constituted a quantum leap in tennis competition. Add Nadal who is a phenom of his own category, and you have your answer, if we are to speak honestly.