That is a matter of scale more than anything else in my opinion. Countries natrually diversify when there are resources to exploit/nurture and even on a small scale they are decidedly not homogeneous. Even when restricted to 13 colonies scale the US would never remain agrarian. The north lacks cash crops and had too much in other resources natrual, labor, and educational to make industrialization nearly inevitable. Even if a fanatical regieme took extreme measures against it they would wind up knocked over by a world that passed them by and wants what they are sitting on.

Not to mention the chicken and egg problems - are they dysfunctional because of monoindustry or monoindustry because they are dysfunctional? Dysfunction leads to monoindustry since the base cannot be lost to war - see conflict minerals. Everyone wisely stays away and refuses to trade with smash and grab warlords and despots out of self preservation alone - owners wisely do not want to legitimise that sort of capital seizing for the same reason one doesn't try to work for a non survival desperation cannibal. They did it before and will probably do it again to you when convenient.

Causation and correlation with such messy subjects that I can see your point even if I disagree with the cause.