The parent's "them" is not your quote's "endangered spaces". There is no plan to fill the carrières/catacombs (the parent's "them") with concrete and filling parts of it (your quote's "spaces) has been an established practice for decades, either for structural reasons (new metro lines, etc.) or for policing reasons (preventing people from entering the system). It was really hard 30 years ago to stay underground for more than one hour without having to squirt your way above a blob of concrete.

Yes, it progressively makes parts of the system harder or impossible to reach and something is definitely lost in the process for the very few who care but that's how the underground in general—and this underground in particular—has always worked.