They make it sound like if Federer 2011 had started losing to all and sundry players. Most of the time in 2011 he lost to players who were clearly superior from the pack he used to defeat in 2005-06. In 2011 he lost in AO to Djokovic, FO to Nadal, W to Tsonga (well not from ATG league but Fed had lost to him earlier as well) and USO to Djokovic. The truth is that Fed had never had multislam season beating this level of players. It is stupid to conclude based on this evidence that he started losing cause he has started to decline. He had never triumphed this kind of competition. All these decline theories are bogus. Did Djokovic decline in 2013 or 2014 ? No, he was out of form or failed to maintain level. If had declined in 2013 or 2014, we would not have seen 2015. All these theories of peak - non peak are figments of imaginations of fanboys to console themselves on defeats of their favs. These are great players and with time they invent new tool while letting go older tools. That doesn't make them any weaker. When they become weak they start on one way journey towards oblivion. See what is happening with Ferrer. That is decline. What happened with Rafa in 15-16 is being out of form when you lose to all and sundry and failed to make a single SF for two years. Same is what is happening to Djokovic in 2017. It is not decline when you losing to just top two players (who happened to be from the level you never dominated ) but defeating everyone else.
Anyways, if one has to believe in theories of peak or non-peak just to keep themselves happy, go ahead and do that. World is yours.