You are obviously interested not necessarily in 'truth', but in postulating that Federer's prime performances have been since exceeded by other players by pretending his play is ever improving while he wins less and less, the implication being that more and more players are surpassing his peak levels. It is extremely difficult to take this seriously, and your appeal to a lack of available direct measurement in an attempt to immunize your position by rendering it objectively unfalsifiable does little to change this, because people have eyes and there comes a point where distinctions are so obvious that mere opinion is more than enough for pretty much everyone. Like my opinion, without weighing them, that a lion weighs more than a grasshopper. Care to contest that one?
Using Roger's reference to his own abilities is simply a severe case of confirmation bias and you know it. You've been flaying at this dead horse for what feels like years now and the argument from authority hasn't exactly grown in stock over that time. If the lion could talk and told you that the grasshopper weighed more than him, would that twist your arm? I acknowledge Roger has adapted some aspects of his play and stayed very relevant at the top of the game, and good on him for championing his skillsalicious, but the idea that he hasn't declined in various faculties is quite the claim.
For what it's worth I'm not intending to single out Roger; the decline is also evident in the other big 3 members. Age is fairly indiscriminant, it's just that Roger is significantly older and appears to have declined more at this stage. I expect Djokodal to be further declined by the time they are pushing 40 (if they're still playing then).