Why do we have a desire to restore some old things to mint condition, while we prefer patinas on others? By extension, who gets to decide that one building is ruined and subsequently condemned, while another is elevated to the status of Ruin that needs to be protected and preserved? My best guess is that we have an emotional attachment to how some things come to be in their current condition, while for others we don't.
The fact that a patina can't be applied in one sitting makes it special. It represents extensive usage by multiple generations. Old stone steps with minor erosions from usage or brass banisters with discoloration feel classy and worth keeping as is. But, a run-down 70's-style apartment building feels less worthy of preservation. Who knows, maybe we'll wish that we'd kept some of them around.