This is just a thought to help with servers that are heavily being dominated by one faction.
Add a protection cost (to deal with the corruptions outside the settlements?) that will continue to rise logarithmically. Eventually it will cost too much to protect the city and the owning Company will have to let it go.
Once a settlement abandoned, it will turn white allowing the other factions to fight for it. The previous faction will have a cooldown to allow other factions to have a chance and allow them to take part at attacking in a war soon following (instead of always defending)