Hmmm...I think I tend to look at this issue differently than a lot of folks because I do build 3d maps for all my tabletop encounters. But I do it by hand, with physical terrain. This is a part of the hobby that I deeply love for a lot of reasons, and I always have.
It takes time to build a 3d battle map and they take up a lot of the tabletop. So I mostly only do it if a combat encounter is going to happen there. I could see the VTT being similar, where you have a 2d map for navigation, etc., but then switch to a 3d map when things happen.
Another thing about using a 3d battle map is it makes me want to railroad the party towards the encounter that I have spent so much time building. I really have to fight myself on this, and sometimes they make different choices and we wind up doing theatre of the mind or drawing things out with the old dry erase markers and flip mats. Also, I usually prepare one or two extra 3d maps to cover the most likely options but this means doing a lot of extra work that doesn't get used. This would probably be a lot easier on a VTT, especially because I bet I could save maps for use at a future time (you can do this IRL and in fact I have one such map tucked onto a shelf right now, but it's a hassle).
A great thing about a 3d map is that it helps the players think outside of 2 dimensions. I have found that they are far more likely to make interesting tactical choices when they can see the stack of barrels in relation to the ledge and the cage dangling high above, or whatever. So it makes combat more exciting, which it needs. The VTT might provide this as well, though I am not sure if it will be as effective as looking at the physical terrain in front of your face, being able to grab and move your own miniature, etc.
Storage is a problem. My spouse is getting a bit annoyed with the amount of garage shelving that is given to my plastic bins of terrain and miniatures. A VTT solve this problem, completely.
Cost is a huge issue. It costs thousands of dollars before you have enough terrain and miniatures to start being able to handle anything like the variety of situations that a D&D campaign gets into. I know folks are worried about the cost of a VTT, but I can pretty much guarantee that it will be minuscule by comparison.
All that said, I like the idea of the VTT. For new players, in particular. But I don't see it doing that much for me. There is a magic to real, tangible objects that I can't imagine a VTT will ever give me.
