This is survivorship bias in action. They intended to build a bridge. That is all we can infer from the fact that a bridge exists. That they used available materials and the bridge was not abused by subsequent usage outside its design spec is not proof of any specific intention on the part of the builder. That exists only in your head. It's like assuming dinosaurs died in specific spots with the intent that their bones petrify and fossilize. You're reading too much into not enough facts.

Maybe, but lots of roman buildings endured the time in much better shape, than many buildings that were build after them. Have you seen some of them in front of you? I have and I am impressed. (I am in italy right now to go look at some more).

Also we know a bit more about the romans than just their bridges.