I read an engineering guideline on concrete that I wish I could find. It compared the economics of building a steel vs reinforced concrete bridge. Steel came out better when you considered the lifetime and especially the recyclability, but concrete was cheaper to build initially. The steel bridge would last indefinitely, as long as you kept it painted, or even replaced rusted parts.

Reinforced concrete is costly to demolish, and nearly impossible to recycle. Crushed concrete can be used as a filler, but in order to be used for fresh concrete it must be free of contaminates, which is rarely the case, and no one wants to risk the integrity of a new structure of any importance.

The percentage of all concrete structures even built, that are still standing must be quite large. This will not be a great legacy to leave future generations.