For all of his considerable skills, and the excitement he brings to the races, Verstappen has had somewhat uneven results so far in 2017. He's been involved in four first-lap crashes, and he still trails Red Bull teammate Ricciardo in the points.
Verstappen and Hamilton on the same team would be a recipe for disaster. Because the absolute last thing Mercedes (or indeed any team) wants would be their two drivers taking each other out racing for the first corner.
Verstappen is still incredibly young. He's incredibly talented. But IMHO he still seems not have acquired the judgment and maturity that pretty much every F1 champion has to have. Skill, bravery, and aggressiveness can win a race against the odds. But inevitably the odds catch up with any driver, and are likely to result in DNFs, crashes, and penalties that will put a Championship out of reach.
Max Verstappen is obviously good for F1 and its fans. And maybe by the 2019 season Verstappen will have developed some of the maturity and judgment he will need to be a Champion.
Whatever your feelings, I think we can all agree that F1 has made great strides towards becoming more competitive and more interesting to watch. Lets hope that one or two more teams can rise to join the ranks of those with a reasonable shot of winning on any given Sunday.