Zverev's case is a tricky one. He surely has put himself in a solid position, and probably has more big titles than the rest of the field (apart from big 3 +1) combined. It is impossible to ignore that. Being young, and supposedly on an upward curve, it is irrational to bet against him in the future. He does not have much supporters here, but one reason is that he is kind of old news on this board. He was in the conversation as early as 2015. When he got to top 10 some folks here were tired of him already...
Other thing is that he is becoming more and more pragmatic, which, results wise, works wonders. He was "flashier" when he was (even) younger, just check the highlights of his famous Nadal defeat at AO 2017. He is now a solid player, with a close to spectacular back hand (to change the ball direction like he does is extremely difficult) and with a very good serve. Being a good mover (extremely good for his size), volleying better by the day, he is bound to good results, if his head does not get in the way.
I won't go as far as predicting majors for him in 2019 -- his results so far speak too loud against him on that regard. I am not saying that there is necessarily something inherent there holding him down -- it is hard to tell, but at the very least there is some mental hurdle to overcome. Deep in a five setter, exhaustion grabbing you by the throat, this is hands down a factor. So my take is that he breaks new ground on majors in 2019, but will suffer a bad loss in a quarters or a semi before putting himself in a final.
Put this all together, he is a good candidate for YE #1 -- but without a major under his belt. In this case we will have to read countless times people saying he is not a "worthy" #1, something which I can tell before hand that in ANY case is a giant stupidity. My already large "ignore list" could grow even more if that happens.
While I do not root against him, I surely do not root for him anymore (as I did when he was around #50). I guess is those horrible necklaces.