Contemporary historic preservation sees itself as the guardian of historical substance. The content of a monument is bound to the preservation of the inherited material.

Georg Dehio’s principle of "conserving, not restoring" is often invoked as a synonym for this self-conception. Old and new need to be clearly separated.

It is a counter-movement to the 18th century historicism which ”destroyed” a lot of old monuments beyond repair.

Personally, I think we went too far on the conservation angle (at least in Germany, not sure about other countries), and should restore a bit more again with the knowledge we have. But much more intelligent people have debated that for centuries, so I guess their answer would be the same like https://askastaffengineer.com/.