I think that the title of the thread points a finger, yet the information Gary provides is objective. Somewhere in the thread (iirc) he also states - and of course, I agree - that tennis isn't swimming, track and field or bowling. It is hard to have pure numbers that aren't dependent on competition.
Because of Gary's research, I have thought more about the 60% (games won) benchmark as an indicator of dominance. Obviously, if a player is only holding serve 60% of the time, he'll need to break serve 60% to achieve that. If he's only breaking 20%, he'll have to hold 100% to achieve it. (Exaggerating to make a point.)
Of course, true break numbers usually fall in that 30-35% range for top players -- with variations for surface and conditions.
I accept that Novak's slight physical drop (decline sounds too precipitous) will show up in his defensive stats which I haven't pored over. But he has also kept in such great shape, and he has compensated often with better serving and a more aggressive forehand. I'm not sure what the stats say, but he also knows how to peak for slams (and only for slams) and often gets better as the slams get deeper. So while Novak is hardly immune to inevitable physical "decline", at four tourneys a year (assuming ability to enter them) he presents nearly the same overall challenge as he did in 2011 and 15-16. His formula is just a little different.