I had the same thought, but actually the super long timeline could also be evidence that they're taking it seriously. Roppongi Hills, another Tokyo development, took about 18 years from the start of planning to the end of construction.
The reason why Roppongi Hills took 18 years was because much of that time was spent acquiring lots from 400 smaller property owners.
The construction of Mori tower itself took ~3 years.
On the other hand, if they are serious about it and haven't decided on the location yet, it means that they probably haven't started acquiring the land, or that they are attempting to acquire it at several locations.
Given the Roppongi Hills example, provisioning 15 years just to acquire the smaller properties seems reasonable (and completely crazy at the same time, depending on the point of view).
