I don't think they'd let that spot sit vacant very long intentionally. That's prime resort real estate, especially so close to the conference center. I have to believe every day those rooms are gone is significant lost revenue. I can't see them delaying very long.
If this is going to be a regular hotel addition, they would/should put it up right away for all of the reasons you have given. But if it's part of DVC, they will want to control the market so that they don't have too many properties out there for sale. If you can generate demand by decreasing supply, it makes sense for a short time.
Even so, they will announce this as soon as they can. Even with the controversy over the concept and design of the tower (who knows if what we've seen is legit anyway), people are salivating over the prospects of this project. To quote the Fates from Hercules, "DVC Contemporary--It's gonna be big!"
Okay, sort of quote the Fates. . .
Given that the current economy and attendance at WDW remains steady, I would think you would see an announcement soon, but not immediately--less than 18 months for sure, unless there are some major developments in the rumored DVC California project. I would not be surprised, however, if those two projects were announced close together or event simultaneously, since they would be aimed a slightly different market (i.e. west coast vs. east coast).
Again, I go back to my original statement--nothing until AKV is close to being sold. It would not make sense to throw that many rooms into the timeshare market.
I guess what I am trying to say is that you are right--they need to announce ASAP, but they need to wait just a little bit longer.