Sorry mate, but how old are you?
You can't know the future in a such long term. Who knows? Novak could never win another major, he could lose a step in 2 or 3 years and all these players who came close to winning against him (ex Tsonga at RG, Wawrinka in this AO) will potentially beat him.
Federer longevity is an anomaly. It's extremely unlikely that Nole will win 12 others slams.
At Nole's age, Federer had 10 slams and Nadal 11. Murray will annoy him for many years, that's for sure. Tomic can very well improve his game and it's very likely that some youngsters will be able to beat Djokovic when he will be slower. At RG, he will be 26, he's not that young anymore.
I'm not saying ''he will'' but that it is a possibility.
If we assume Nadal is done and Fed won't be a big threat because of age in the future and Djokovic will be healthy for say 5 more years...who is seriously going to challenge him at RG and AO ? Please don't say Tomic lol, these youngsters won't reach that level. Tsonga, Wawa are also getting old. Honestly do you really believe someone will challenge him at RG and AO ? That's 2 slams EACH YEAR where he is the clear cut favorite. No other player comes even close. It's not out of reality to think Nole can't win 6 slams in the next 2 years (if he stays healthy and Nadal is done). Even if he loses out on HALF of the slams where he is the clear cut favorite he can still end up with 14+ easily. A lot of this depends on the competition.