Yeah, bug-in under duress can only be sustained for just so long before an attrition of supplies, wits, and resolve take hold. Zen would be to realize when staying put is no longer as desirable as packing up and heading out.

My whole premise is to try and ward off the first major wave of immigrants/invaders, and hope that subsequent waves are greatly attenuated. However, if the bubble went up and there was a prolonged lack of authoritative response, then I suspect some form of anarchy would be inevitable.

Basically, it is just to try and hold on until the cavalry comes. There's really no such thing as a perfect defense; only a suitable deterrent.