Yeah, pax romana made empire wide trade possible and profitable which meant increased specialization. When trade broke down, most local areas only had rudimentary knowledge for how to make basics (pottery for example) and had to relearn/rediscover or just find new techniques. Which gives me an idea for another way to implement a kind of dark age and make specialists more interesting at the same time. Specialists, become more effective over time but only as long as they're working. A significant population drop would mean losing specialists along with their associated knowledge. How fast a new specialist would improve would depend first on existing specialists in the city (direct training) which would be fast and overall level in the nation (migration, training, etc.) which would be relatively slow. In the absence of any higher level specialist, it would be slowest. Buildings/techs might change this/mitigate loss (libraries, etc), printing press/telegraph/railroad make the national transfer rate faster for example.