your theory, which i was the first to state, was true in 2015 regarding stamina. Nadal's dramatic drop in endurance caused him to lose his famous mental toughness that year, but he recovered much of his stamina since then and now its more about his energy level (and of course injury) during a specific match. Clay requires more stamina than any other surface, besides maybe a very slow hardcourt, so its makes little sense that you now differentiate clay form any other surface regarding playing Federer or Djokovic now from that standpoint.
However, I do agree that Nadal should not try to 'out-aggressive' Federer or Djokovic now, especially Federer with his new slash -n-bash method of playing Nadal recently. He's even against Djokovic atm and anybody who cites the AO final as proof otherwise is being foolish, Nadal was injured in that match. their Wimby semi is a much better reflection of their current match up (a toss up with Djokovic having a slight to moderate advantage on HC (depending on the conditions), both mostly even on grass, and Nadal a favorite on clay.
Now vs Federer, Nadal needs to change his tactics! stop with the artificial aggression, change up the pace and speed of the game with more slicing and moonballing, keep the ball deep, and stay aggressive with his serve.