It's a small city, so if it's inactive for 30 days, then you'll see a new immigrant.
The program doesn't seem to worry about the state of the inactive cities, merely that they're inactive. To do otherwise would require every city to be looked at every day, rather than just looking at the last log on dates older than 30 days, and flagging those cities for replacement if they're neither Premium nor haave an average development of at least level 4.