I believe that's different. The top 29% of the Burj Khalifa is unusable by design -- its only purpose is to make the building taller.
I believe the OP's point is that when usable space in a building goes unleased, it makes it hard for the investor to turn a profit. Therefore there is no economic incentive to build a taller building.
I find this argument a bit hard to understand for another reason, though. If there is no demand for space, then why does anyone construct new buildings, and why is rent so high in places like NYC? It seems to me like there is actually incredible demand for space in dense cities.
Many skyscrapers are novel- there is a certain prestige, and thus premium- to having an office in the Sears tower or the Burj Khalifa.
That is added on to the fact that skyscrapers are also engineering marvels, and the cost per unit goes up quite a bit once you have to start dealing with those problems, compared to the much more standard 6 story building as an example.
An additional novelty factor is that actually supporting the weight of the tower will require different techniques (and significantly higher costs) depending on the type of land the desired build site happens to be on.
All of these things create something of a price floor for rents.
