You're right to not buy this entirely. Roman concrete was great because it could harden while in water. And waterworks was probably the primary reason why Rome

You're right to not buy this entirely. Roman concrete was great because it could harden while in water. And waterworks was probably the primary reason why Rome was such an early superpower (for multiple reasons: hygiene, stronger ports and also mechanical force, as "free" energy give you productivity increases. But it had less tensile strength than our current concrete. The reason why multiple structures still hold today and will hold longer than most of our structures is more simple than that: they over-engineered the shit of everything that still hold today. Also for the same reasons, multiple bridges, castles, church or structures built between the low medieval era and high med/renaissance will likely last as much as roman structures did.