There must have been several dozen people on these boards alone predicting 16+ major titles for Djok. That would mean at least a few in his 30s. With your "many" were you allowing for even more than that?
I think I was at the point where I gave him something close to a 50-50 at getting to Fed's 17. Maybe 40-60, can't quite remember.
What I do remember is that I came up with the list below, every time people said:
But you don't win slams in your 30's.
I.e. look at who's Fed lost to since turning 29 (Djoko's age at the FO)
US 2010, Djoko (Rafa waiting).
AO 2011, Djoko
RO 2011, Rafa
W 2011, Tsonga, D + R waiting
US 2011, Djoko, Rafa waiting
AO 2012, Rafa, Djoko waiting
FO 2012, Djoko, R waiting
US 2012, Birdman
AO 13, Murray, Djoko w
FO 13, Gulbis
W 13, Stak
US 13, Robredo
AO 14, Rafa, Stan waiting
FO 14, can't remember
W 14, Djoko
US 14, Cilic
AO 15, Seppi
FO 15, Stan
W 15, Djoko
US 15, Djoko
AO 16, Djoko
FO 16, NA
W 16, Raonic
Now take out Rafa and Djoko - two ATG's in their peak or prime in the respective years and matches.
The bold ones are tournaments where I would favor Fed with no Djoko and no Rafa.
That's 8 slams.
Then I've underlined 3 additional. AO 14, where Stan would have been a handful, AO 12, where Murray would have been a handful (but still maybe not quite mentally ready to beat Fed in a slam? He had just hired Lendl) and the FO 12, where Ferrer was in the other semi, but where Tsonga would have been the semifinal opponent had Djoko not gone through.
Of course, there are so many what if's in this whole exercise, that it's kinda pointless. Would Fed have been motivated past 20 slams? Would Murray have won a slam sooner and have become mentally stronger as a slam opponent sooner? Would others have risen to the occasion?
Nevertheless,
between Fed turning 29 and now, he was beaten 11 times by Djokodal. And far more often than not, he was the third best player by a margin in those slams and would face players with way less experience than him in those end of slam-situations.
Hence the reasoning - with no one near the quality of Djokodal age 23-29 quality to challenge Djokovic, who was left to beat him? He owned Murray 10-1 since the back operation at the start of the year, Stanimal only requires one slam a year (and is 2 years older) and Fedal are both past beating Novak at a slam. Then you have
the lost boys, i.e. generation 89 and above who are yet to win a single Masters, Slam or WTF-tournament. Not a single player younger than 28 has won any of those tournaments. That's what I call and called a
golden window of opportunity for 29+ ATG's.
All in all, even if Novak doesn't age nearly as great as Fed, he had and has (provided he can recover his form, motivation, mental capacities or whatever is missing) a golden window of opportunity. Murray has gotten better compared to 2014 and 2015, surely. But not to the point where I would pick him over Djoko outside grass,
if Djoko can find something close to his own form. Afterall, both Nishi and Raonic were points away from beating him just two weeks ago.
The post ended up longer than expected, sorry. 
. So as you say, scary how fast things can change.
