Or they could just not do that? That's easily thousands of hours of work, even for that one Korean guy who could smash them out like a production line.
They don't have to. IIRC there were Warband mods which made one or two towns for a faction and used them for all of their cities - makes sieges repetitive. BTW I was referring to different eras in which the native settlement scenes would be anachronistic. I guess a horse and musket mod could just junk sieges and fight field battles to control settlements.
However, the best initial option IMO remains make-overs. Far easier to take say eight kinds of native factional buildings and substitute new models with exactly the same footprint and game id, so they're substituted in every vanilla scene for that faction. Building heights will cause problems with archer navmesh positions on rooftops.