Roman construction was also much less efficient because they had no material (besides wood) capable of carrying load in tension. Rebar allows us to make cheap practical structures that are impossible with just concrete - roman style or not.

It would be quite fascinating to see what kind of structure we could produce if we decided to make the longest lasting cement structures we could create with modern technology, and assuming minimal maintenance over the lifetime of the building. A one-and-done kind of structure.

I bet we could do fairly well. Hundreds, maybe even thousands of years. We've learned a lot about how to form exceptionally long lasting cement. We just choose not to do it that way, most of the time.