It’s even more amazing than that in the case of Rome. To cite just one example, by the time of Constantine I in the mid-300s CE, Rome could support armies totaling 650,000 men. The logistics and organization required to do that are staggering. After the fall of Rome, it would take until the time of Napoleon’s Grand Armee in the early 1800s before numbers like that were fielded again. Even today, there are relatively few countries with an active military force of that size. They weren’t just sitting around either. Rome was always fighting someone. It speaks to the ability of ancient peoples to organize and support truly massive endeavors and sustain them over literal centuries. I mentioned Napoleon’s Grand Armee earlier. It was large, but it only lasted for about 5 years.
So, yes, a ton of technology was lost for a long time, both physical and social/organizational.