I don't think the placement of this would really factor into the decision for them. It's more a question of being able to get some press coverage for a 'relaunch' of Epcot that includes a mention of some kind of new attraction. Without Journey of Water, they don't really have anything of note to promote when they finally finish.
The issue of when all the other work will be finished is an interesting one. Is it really, though, beyond the realm of possibility that the rest of the work could be finished within the next few months? We've become accustomed to everything crawling along, but the new building is now largely enclosed and they seem to be up to the point of constructing the walkways and landscaping the main hub area. If they wanted to, I'm sure they could open this up by September/October if that's the plan. Wasn't that in the vicinity of what they announced for the completion of this project?