The immediate question, of course, is how long will it take for everything to be destroyed? What will the anti-griefing measures be like? For example, if a monster can destroy a city, I foresee someone kiting a monster to the city at every opportunity.
It's kind of sad how everything can turn into a tool for griefing. Destruction can lead to griefing. Creation can lead to griefing. For example, people can take up all the free space with buildings, making it hard for new players to start. Or they crowd out existing players.
I think there could be a lot more work to be done with costs in MMOs, especially non-linear costs. For example, maybe maintaining 5 buildings costs 1 resource per building, but maintaining 10 buildings could cost 2 resources per building. Or something where it's easy to destroy one piece of terrain, but gets harder and harder to destroy more terrain.