I seriously doubt a building made of scrap metal, nails, and leather would last through the winter. Upkeep for things is necessary for two reasons A. Nothing can last forever B. its a resource sink. It keeps the economy chugging along.

Everyone makes upkeep out to be this tedious job that must be done every hour of every day. If players can make quests, then there could be upkeep quests. These quests could involve one structure, or multiple. It could be the architects job, as an engineer, to check each structure to make sure its well maintained. Especially after a siege (provided there are sieges)