Well you're right that it is too soon to say on Thiem, Kyrgios and Zverev, and I would take it a step further: it isn't even certain that they'll be "near-greats," although all three are very much on that pace and Zverev is, of course, still safely on the "Pace of Greatness." But I mentioned those three because, as of this moment, they're the most likely to be elite players of all players born in 1993 and later. As I see it, at least. There are plenty of other candidates: Pouille, Fritz, Coric, Tiafoe, Rublev, Kozlov, not to mention Shapovalov and Aliassime.
(And by "elites" I mean something a bit different than greats, probably including both greats and near greats - players that are top 10, occasionally top 5, and at least dark-horses for Slams)
I also agree with you that there are no surefire true greats in the next generation, that they very well could be a generation of several multi-Slam Slam winners but no clear 6+ Slam greats. But it is too soon to tell. As I wrote in one of my pieces, Zverev and Fritz look especially promising among those born in 94-98. I'm also curious about Rublev and Kozlov, and the two young Canadians (Shapo and Alia).
It is also worth mentioning that in the entire history of tennis, going back to the late 19th century, the biggest gap in birth year between 6+ Slam winners is the ten years between Sampras (1971) and Federer (1981). As Novak was born in 1987, a gap of the same length would be 1997, which is conveniently the birth year of Zverev, Fritz, and Rublev - three of my candidates for the next great player. If we expand that a bit we get to Tiafoe, Lee, Kozlov, and Tsitsapis in 1998, Shapovalov in 1999, and Aliassime in 2000. It would be a massive historical anomaly for the next great player not to be born in at the latest the late 90s, and if he was born in 1998 or later it would be the largest gap in tennis history.
I still hold out hope that Zverev--and possibly Fritz--could break through as a true great. We must remember that these guys are only 19 and 18 respectively, so there's still a lot of time. Zverev in particular seems really on the cusp. He's ranked #27 now and, I suspect, will win his first title before turning 20 in April of next year, and make his first Slam QF no later than next year. But I think it won't be until 2018 that he's really starting to enter his prime, Fritz then or a year later.
As for your question, my first response is that there are TONS of comparable players for him using those criteria. Raonic is far off the "Pace of Greatness" that I outlined, but he still is very much on the pace of "near-greatness" (for 2-4 Slam winners).
It is difficult to compare contemporary players like Milos to historical players because it seems that players are blossoming much later now. Part of this may be that we just haven't seen a true great since Novak, and that we'll know the next true great when he rises into the top 5 as a 21-year old like all true greats have (which is, I think, the most stringent criterion of true greatness). In other words, it may be that players actually aren't peaking later, but just that we haven't seen any true greats since Novak and before him Rafa, both of whom blossomed within the normal young range for true greats.
One of the statistical differences between true greats and near greats is that they meet a lot of the same benchmarks, but the latter do so at an older age. So while all true greats reached the top 10 as 20 year olds, Milos did so at 22, and all true greats reached the top 5 at 21, Milos did so at 24.
Aside from statistics, the player that Milos reminds me most of is Goran Ivanisevic. But his career is quite different in that Goran--like most of his peers--broke through at a very early age, finishing his age 19 year (1990) at #9. But it took Goran until he was 29--and well out of his prime--to win his one and only Slam, because he peaked alongside of Sampras and Agassi; I suspect Milos will win his first before that because the true greats of his era are a few years older, and he may win two or three. The context will open up for him, as you say, both because the current elite are in or entering their 30s, and because no dominant players have yet emerged from the next generation.
2018 should be an interesting year. Consider the age that various players turn:
Rafa turns 32, Andy and Novak turn 31, del Potro 30.
Milos turns 28 at the end of the year, thus will be 27 for most of the year.
Thiem turns 25, Pouille turns 24, Kyrgios 23.
Zverev, Fritz, Rublev all turn 21.
Shapovalov turns 19, Aliassime turns 18.
So you can see that 2018 will see the older generation solidly in their 30s, Thiem, Pouille, and Kyrgios all very much in the heart of their prime years, and the younger guys entering their prime, with the following generation starting to appear on the radar. But unless we see one or two of those younger guys break through in a big way, it will be an open context in which players of a wide range of ages could win Slams. Should be exciting to watch!