Stagnation of Technology?

The problem is that you're using the exceptional to describe the norm. 

Watermills? A Greek invention from, oh would you believe it, the 3rd century BC. Philo of Byzantium wrote about it in his Pneumatica and Parasceuastica,  though whether he invented it is another matter. Like all clever ideas the Romans exploited them here and there - but please notice water mills were not a common feature of Roman rural industry. Like all such things, they were individual, localised, and not well known. I know of a three bladed water powered stone cutter the Romans built. Just one. This is what I'm trying to get through to you. Rome did not communicate ideas. Where are the books on sciences written by the Romans? They even refused to believe Pytheas when he wrote about frozen sea ice.

I hardly think the Pantheon was 'science fiction'. One single intelligent use of concrete in the entire history of the Roman Empire does not progress make. It represents an achievement of one talented individual., not the society around him.

We landed on the Moon back in the seventies. Why not take a vacatio0n out there? Oh, you can't, because the existence of such technology is individual, localised, and not well known other than by concept. The existence of rockets does not make us all rocket scientists.

Sorry, but discussing technology and the Roman Empire in the same sentence is a contradiction in terms.