> If there's anything worth defending, people will volunteer to defend it.

That's a pretty binary view of the situation. It's not enough for something to be worth defending: People have to recognize that it's worth defending — and that might not happen instantaneously, because "people" come to appreciate things at very different rates.

What if people do but not in enough numbers? Farewell, sweet country!

Indeed, and there are many historical examples of this.

Today’s average Western nation-state tends to be a rathole that spits into the faces of its citizens every single day, until the moment the state is under attack, at which point everyone is told that they owe their lives to their sacred motherland that has done so much for them.

Seems like there's a clear free-rider problem, the volunteers make the sacrifice and the non-volunteers reap the benefits.