Maybe it will turn out great, and I'm not against that. My questions remains though. Why? Why couldn't they have added all this new stuff to the original Terraria instead of starting a whole new game? Also you mention that no one from 1.3 is working on this. Well if there was no plan for Terraria OW, those devs would be able to work on Vanilla or Terraria 2 (which I don't think is necessary either).
Maybe I'll change my mind as more info comes out, but as of now (yes I know this is pre-alpha), I'm not a fan of the art-style, so much so that it would put me off from buying the game entirely. Also, I just don't see why all this new content couldn't have been added to Vanilla. If some physics engine or whatever poses limitations, fix it. If they want to add a new game style/mode, go for it. Just don't make a whole game very similar to the original but with new objectives. I don't wanna accuse the devs of anything they aren't guilty of, but to me it seems very profit-motivated. Change the look, add some new stuff, add motivation (?) for cleansing the world. Boom, new game. I'm sure that's not the reason this, but I have reasons to be suspicious (remember the whole only-developing-Terraria-for-the-profits drama that came up after 1.1?). I'll say it again, because I know someone will try to accuse me of not saying this. I highly doubt money is the motivation behind creating this, but I wouldn't go full dogmatic fanboy either.
In the end, I just think this (and Terraria 2) is unnecessary. Don't have anything against changing game mechanics or art-style (not against changing it, but I am against changing it to the current state); I just don't see the point.