Idea of a city in the sky is rather interesting. But back to reality, can such a structure be supported sufficiently by its foundation? Even if the moment of inertia it has is very high. Moreover, at higher altitude, the building will be subjected to even stronger winds.

Of the proposed alternatives, i suppose underground city (not the underwater version) is more viable. But like its counterpart submerged underwater city, it has the problem of ventilation and sunlight.