Hard to say. There's still a sizable gap between #3 and #4 which would require a few more years of a high level from Novak. Maybe 3 Slams? If he gets to 15, he's at least in the top tier. If he wins five or more Slams, wins another year end #1 or two, to go along with the 40+ Masters and 80+ titles he's going to finish with, he's probably #1.
The problem with passing Rosewall is that he accumulated such a deep record: 133 titles and 23 Slams (including amateur, pro, and Open Era). Rosewall was probably the #1 player at year's end only a few times (3, according to Tennis Base), but he was one of the five best for about twenty years (19, according to TB...which is two more than #2 on the list, Tilden at 17, and six more than #3, Federer).
And of course it is hard to compare the context of the mid-50s to mid-70s. One thing that cannot be understated about Rosewall (and Laver) is that they adapted and excelled in whatever context they played in.
In the end, though, I think the tiers are more meaningful than the exact rankings. The first tier are the GOAT candidates. The second tier are the inner circle greats that are just on the edge of GOAT candidacy but missing some ingredient. The third tier are true greats but are clearly not the GOAT, for whatever reason. The fourth tier are the lesser greats, and the fifth tier are the borderline or near-greats.
