First question would be whether that would be realistic. If it is realistic on certain conditions, such as extremely happy population, effective administration, no foreign meddling into the cities affairs, etc. then it might be an option to do so.

But I would think that is rather the exception.

Non-retroactive means it will only work with new campaigns. Of course, you could manually edit the script (or use the console) and reset the interloper trait each and every turn for that character.