Innumerable "medieval" pictures show the founders and benefactors of buildings with their projects: these are normally quite complete. The Limbourg brothers show Notre Dame de Paris and a whole series of royal French building projects in 1415 perfectly finished and bare of even a builder's plank.
The reason most Gothic cathedrals remained incomplete was that the societies that built them stretched their resources beyond their limit. While this sometimes proved an excessive service to a theocratic ideal, it held ambitious societies together and supported the political and economic ambitions that made Europe a dominant world power. It had nothing to do with a conscious desire for incompletion.
The rejection of the spiritual and symbolic aspects of architectural function should not be abandoned out of whim or ignorance as Brand invites us to do, nor out of the cheap sentimentality of 1980s supermarket monetarism. If any social project is to make us think effectively about our ecological destiny, it will have to espouse the same extravagant and sometimes inconvenient values as the cathedrals of the past and the Sydney Opera Houses of our time, finished or not.