Villagers currently do not rebuild broken or improperly spawned structures.

This includes structured destroyed by creepers, those spawn in an unusable state, and those that are destroyed by bad-mannered players looting the village.

This leads to an eventual decay unless players intervene and rebuild villages.

The best proposed solution is to make villagers rebuild structures.
This could be expanded to add a feature will villagers would build new structures as the village grows, to remove the need for population caps.