He did decline, but still good enough to destroy anyone who isn't named Nadal on clay or Federer on hard/grass. And he will keep doing it for at least 5-6 years from now. While this generation of young players is promising and no doubt they will win slams when the BIG3 retires, they are just not good enough.
He tries to adjust to his current situation, he is not physically as good as 2011 or 2015(although he is not far off), he improved his volleys and his strategy and became mentally stronger. He surely declined in some aspects, just compare him now to his level in 2011, but just like nadal and federer he refuses to fade away and tries to improve what he can.