Then make corner posts repairable. The fact that they are not is just another example of what I was saying regarding the disparity of power.
Its almost like raiding a village, pillaging or destroying hundreds of hours of work, and potentially driving paying customers from the game should involve a little effort and sacrifice on the part of the raiders. Happy fun town is going to spend hundreds of hours rebuilding if they do so at all. I really couldn't care less about the 12 hour time investment of raiders.