[I_complete_me]

The subject is complicated. For example see this paper[0]. The setting of concrete goes on for a long time and the Romans were probably not aware of how long their concrete(s) was/were going to last. There is a type of 'survivorship bias' going on here. But still, I guess, they knew something important. Adding reinforcement to concrete may well be a source of shortevity - or what ever the complement to longevity is.

[0] https://www.sciencedirect.com/science/article/abs/pii/S00225...

[UncleEntity]

It’s not like the Romans started out in a place where there weren’t thousands of years old structures nearly everywhere they went.

Assuming they didn’t want their structures to last as long as the ancient ones (at their time) is just laughable.