Taking time to complete the project to spread out the capital expenditure makes no sense. Most of the costs will be depreciated over the anticipated life of the project, so it does dot matter if it takes 1-3 years to build, the cost is spread over 20 years. It is in Disney's interest to complete the project as quickly as possible so they can start charging rent. The faster they get revenue, the better the ROI, the happier Wall St wil be.
The two things driving this time line are the logistics of building while stores are open (at least doubles the timeline) and minimizing construction costs. Anyone who has ever done a major renovation to a house knows it takes longer than building the original structure. You might be able to do it quicker, but the costs rise exponentially as you compress the time and if anything goes wrong, there is no slack in the schedule to correct it.