Sure, but the fact that some monuments have held that well is impressive. It might have been a happy accident, it most likely was, but the valuable insight her

Sure, but the fact that some monuments have held that well is impressive. It might have been a happy accident, it most likely was, but the valuable insight here is that we know that there is a recipe for concrete that lasts for thousands of years in real world conditions. We probably can do better today, we have a much better understanding of chemistry and computer simulations, but what we can't do is conduct a thousand year long experiment, and that's the Roman empire have done to us.