At this point, it sounds like the broadcasters are moving ahead full steam.
We're seeing a lot of chatter from a few on the scale of Sinclair and PBS, but not much from the others. I don't think it is fair to say that they're all on board.
The only potential failure I could see would be not enough buy-in from consumers.
This is a show stopping failure should it come to pass and it is entirely possible absent a mandate. It doesn't matter what the technical merits of Next Gen are if the OTA hold-outs aren't interested.
Heck, some people are buying "tuner free" TV's and completely oblivious that OTA exists at all. The broadcasters are really going to have to crank up their marketing game once they get 3.0 going.
It doesn't seem likely that those who have lost interest are going to rediscover OTA; especially if they can't conveniently consume it on the devices that they now use to view content. This is not necessarily a
Field of Dreams as consumer intertia is a formidable force; especially if there's little compelling on the other side (see more at Ultra HD programming choices and successful applications of interactive programming).
The cable-based carrier lobby is a force that must not be dismissed as they have a lot more customers and plans to use other technologies to deliver the tasty bits that will attract customers. Customers don't care about advertising opportunities and bandwidth savings as long as they get the content they want.