Of course, this has to be balanced with some amount of excavation to actually improve those methods, and with enough information being gleaned today to keep us interested in actually funding the practice of archaeology for tomorrow.
They flattened a few city blocks to excavate it.
But they will never flatten the cathedral, or other large famous buildings in the vicinity.
Point is: the longer we wait to excavate, the more the likelihood that something's been constructed on top.
But secondly, as long as the building doesn't destroy the site (say, the ruins/remains are buried deep enough), then there is no problem, and the logic still applies: it will be found some few decades or centuries later, so chances are there will be more that can be gleaned from it then.