It's technically doable. They closed Jaws in January 2012 and Diagon was open 2 1/2 years later. Using that timetable they could wait till as long as till after the Holidays and do 2021. Now will they, or should they is a whole other question.
One thing to consider. I really can't see them giving all LC to Zelda anyway. It's around 7 acres. That's huge. Thats bigger than DA, Hogsmede(minus the DC plot), or depending on what you count as usable area it's even as big as KidZone. No way Zelda deserves that much land. If it's going to LC at all, I think they split it and do part Zelda, and part something else. Now does that mean more Nintendo, Seuss expansion, a Kong-like situation for a one off ride/area, or even just kept partially LC till another idea comes along. So if you only do part of LC, then the timetable isn't as tight.