Oh, the dark age myth revisited. Our bus stops currently show some nice ads about it and how the reality actually looks like (To get more support for maintaining historic monuments, so everybody is able to see the reality before it becomes legends and fakes).
The people didn't forget how to build aqueducts. The economy simply broke down with the Roman Empire to afford maintaining the large roman ones, but there are many examples of smaller ones, build more economic. Also many richer cities kept the Roman aqueducts maintained until the 15th century (like Rome). Once the economy kicked up, the middle ages even exceeded by far, what the Romans (including Vitruvius) had been able to build. Also technology never advances linear, it often needs advances in other fields to follow, to make the next big step. Just to stay at the aqueduct example: To distribute the water in the cities, the Romans needed vast amounts of lead, about a ton per 40m water pipeline. You need a certain economic power to produce that. And for getting this production, you need to be able to produce large amounts of metal in general.
Also, contrary to the less popular misinformation in bad science TV, the Romans had not been able to build their aqueducts with a standard grade. The grades varied a lot, every building had a different one. But the buildings were build pretty accurate, but thats not something, only the Romans could do.
And the same with spaceflight. It doesn't stagnate at all. It just advances in other fields. Apollo was just a faster shot at the moon, afforded with a much bigger investment into spaceflight than before or later. Keeping this rate of funding would not have gotten us further at all, it would maybe stagnate at the moon, but it could also still have been smarter to focus on LEO.
