All true, Great Hands. It really is a sands-in-the-hourglass thing. Novak has to win six (or seven) more before the 1994-98 generation comes of age. Using my generation theory, here are the potential obstacles:
1979-83 Gen (Federer, Ferrer, etc): No real obstacles. Fed can't beat Novak at a Slam anymore.
1984-88 Gen (Novak, Rafa, Andy, Stan, etc): Unless Rafa rediscovers the fountain of youth, he's no obstacle. Unless Andy gets a new brain, he's not really an obstacle. Stan is always dangerous but is erratic. No one else is really a serious Slam challenger (sorry, Juan Martin).
1989-93 Gen (Milos, Kei, Grigor, Thiem, etc): This is the group that should be dominating the field right now. As I wrote elsewhere, if you go back and look at the year-end rankings in five-year increments--2015, 2010, 2005, etc--the #1 player in the world has always been in the age range that this generation is currently in, at least going back to 1975. Conclusion: it could be the worst generation in Open Era history, or at least since Arthur Ashe's weak generation (b. 39-43). Milos, Kei and Grigor could theoretically step up and be dangerous, at least at Wimbledon and the US Open, but we haven't seen any real signs of it - with the possible exception of Milos. The other players in this group that could be dangerous are Dominic Thiem, Jiri Vesely, and Jack Sock, but I don't see any of them being challengers to Novak, at least not until Novak starts dipping.
1994-98 Gen (Kyrgios, Coric, Zverev, Chung, Tiafoe, etc): If there's a new elite player on the horizon, it is likely in this group, which turns 18-22 in 2016. Even if there are no true greats in this group, it should be a lot better than the previous generation. Given that most elites were in or close to peak form by the time they were 21-22, some of these guys might start challenging Novak, but probably not for another year or two.
The point being, Novak's window of opportunity is 2016-17. In 2018 he'll be 30-31, and the 94-98 generation will be 20-24 and entering its prime. I think if Novak wins 6 Slams between 2016-17, he has a legit shot at 1-2 after. But 2-3 after might be too much to ask, so he really needs to win 5 of the next 7 to have a good chance of passing and surpassing Roger.