Reinforced concrete and masonry design are underappreciated disciplines of modern engineering, but their Achilles heel is that reinforcement rusts, rust expands, and expansion ruptures. All at relatively accelerated speeds.
Things like the aqueducts weren't necessarily overengineered, they were just designed (mostly) without quickly deteriorating elements, like steel.
Which is to say, 2000 yrs ago, the design of an aqueduct with a 10yr lifespan didn't differ much compared to a hypothetical one with a 100yr or even 1000yr lifespan. At least compared to how things would be done today.
Much of space design seems to be similar, where the minimum requirements aren't that far off from what seems like excessive engineering. But that doesn't necessarily mean anything was "overengineered".
See also the roman aqueducts. Today we would have used about half as much stone, and they'd be falling apart in our lifetimes. Instead, lucky chunks of them have lasted 20 times as long as anyone ever could have expected to need them.