Small nit not to take away from your larger point - I'm not sure the colosseum is a good example of a concrete structure for comparison to the concrete building

Small nit not to take away from your larger point - I'm not sure the colosseum is a good example of a concrete structure for comparison to the concrete buildings of current age being discussed. The colosseum did use concrete for the arches, but was primarily built of limestone block secured with metal clamping. Then plenty of brickwork was used. Concrete was integral to the colosseum, but (I believe) a relative minority of the material on the whole.