Looks like the current growth rate is almost the same for all of them. So, the current status quo of F > N > D should continue hopefully.
That 2013-16 streak for Roger really hurt. Crazy to think that in 2007 and again in 2009 he was +9 on Rafa; in 2010 he was +15 on Novak!
By Decade:
2000s - Roger 15, Rafa 6, Novak 1
2010s - Novak 14+, Rafa 12+, Roger 5+
I think the gap will continue to narrow, through, and all three will finish in the 20-22 range. I could see (and kind of like) a scenario where they all finish with the same number. 21? Of course if Fedal are both retired with 21 and Novak is stilll plugging along, he will give it his all to get to 22. But if history has any poetry to it, he won't manage and they all finish with 21. Like so....
Roger wins #21 this year or next, then has a farewell tour in 2021, retiring at Basel at the ripe old age of 40. Let's say Rafa wins two more in 2019-20, but struggles through injury in 2021 but then makes one more surge to win #21 at 2022 RG at age 36, retiring while hoisting his trophy. Meanwhile Novak is catching up, but slowly losing ground to NextGenners. He catches Fedal in 2023 with #21, and is determined to get #22 but goes Slamless in 2024 and 2025 and then calls it quits at age 38, also with 21.