Periods of dominance are periods where everything falls into place but competition is key when it comes to acquiring the confidence necessary to sustain a dominant period. Tennis is a very mental sport, you can't rely on teammates or even a couch during the course of a match and when a player isn't confident, it makes a world of difference, even when physical abilities are the same. Without question, Federer's best period was 04-07 but it was due to many factors that together helped him sustain that level - extreme confidence, maturity, physical prowess and hunger to make a name for himself. I really believe his physical abilities were the same in 08 but what he lost was a bit of confidence and largely due to better rivals. When he was beating the likes of Roddick, Hewitt, Blake routinely, his confidence was at an all time high. When Djokovic started beating him, Federer's confidence was shattered. That straight set loss in 08 AO semis was big, i have no doubt Federer would've won 08 AO if it hadn't been for Djokovic. That match was mainly due to a very high level from Djokovic and i think less to do with Federer's dip in level, Djokovic has won 7 AOs and in 08 was in beast mode during that tournament. He then got crushed by Nadal at FO final, another demoralizing defeat. These losses, to better rivals, can completely change the flow of things as when a player is not as confident, it can really make a big difference.
again, what fanboys seem to do is always make excuses as to why their favorite players lose. Federer didn't age 10 years between 07 (when he won 3 slams) and 08, his main problem was the rise of Djokovic and Nadal improving. Same for Nadal, as a Nadal fan, i completely accepted that in 2011, Nadal was in his prime, Djokovic was just too good. Nadal had won 3 slams in 2010 and was in beast mode in 2011, he made IW, Miami, MC, Rome , Wimbledon, US Open finals and only lost to Djokovic in these big occasions. Some were claiming Djokovic was in his head, Nadal tactics were wrong or he was suddenly pas his best.
As a Djokovic fan, i also never made excuses when Stan beat Djokovic. To me, at their best, no player is unbeatable - Federer, Djokovic or Nadal. Stylistically, Nadal gave Federer problems and Djokovic has given Nadal problems. Wawrinka, to me, was stylistically tough for Djokovic as Stan is one of the very few players who can hit winners consistently off deep balls, which Djokovic is a master at doing. 04-07 Federer was beatable, 2011, 2015-2016 Djokovic was beatable and 08, 10, 13 Nadal was beatable...