Would add that it is important to remember that most sites are located in the cities that have been and still are inhabited. Fact that those sites are still present and have not been used to build other buildings or demolished throughout centuries if not millenniums, already says a lot.

Additionally, you cannot keep it all. You have to make choices. Not only because of limited resources but also because otherwise you wouldn't have a city that would be liveable. When you have layers upon layers of history, you have to make choices about what to keep and what not.