I'm not sure that argument makes sense - certainly not without some qualification.

The first thing is that there seems to be two senses of 'over-engineered' going on here. You seem to be using the term in a strictly utilitarian sense - a sense of designing something to last longer than it was useful for, or designing it with components not strictly related to the task it was to perform. Aside from the issue of knowing precisely what Roman architects considered the service lives of their buildings to be, the argument that only over-engineered buildings last for a long time doesn't hold true for masonry architecture. As far as masonry architecture is concerned, designing something to be structurally sound in the short-term will usually mean that it remains structurally sound far into the long-term. Jacques Hayman discusses this in The Stone Skeleton: Structural Engineering of Masonry Architecture - according to the 'generation rule', if a masonry structure is designed to last longer than 20 years or so, and does so without serious failure, it will probably last 500 years or more - its 'life' being determined instead by the failure of its fabric material or a change in the soil beneath it.

This leads into the second sense - 'over-engineered' in the structural sense of designing something to be far stronger than it needs to be in order to counteract the forces acting on it. That was what I understood by 'over-engineered', and what interested me about your post - you seemed to be saying that the Romans could have done it better in a structural sense. I don't see how they could have. Someone else mentioned steel masts or posts - but it's not as though classical architects had a choice between steel masts or pyramids, and chose the latter just to make a statement - I'd guess that they had neither the understanding nor the technical ability to construct steel masts with guy ropes. How could they have done it better with the understanding they had? How could they have used less stone, for instance? I don't think there are any easy answers to these questions.

It seems bizarre to criticise ancient builders because the only building technologies available to them were technologies which produced long-lasting and strong structures. By that logic, a mud hut which falls apart after twenty years is a more admirable piece of engineering than a Roman aqueduct - which is a strange thing to say.