Okay, then there goes a chunk of your population. Which of course includes your workforce because, as said, every man works the mines. So much for productivity. So much for economy. To say nothing of the effects it would have with survivors or elsewhere. Namely that it would galvanize them because now Jack can't hide behind "But I pay you money" or the town criers on the payroll. At some point someone would probably just pull a Jaime Lannister and kill him just because everyone would be better off without a ruler then someone who would commit genocide just because he's afraid they might have a point and a bone to pick. 

Ruling through fear is not a viable method for any long term control. It only takes someone to nut up and show that they aren't afraid. And Jack already [i]has[/i] that someone. 

Soldiers are soldiers and many of the men serving even today are loyal to the country rather than the leader, regardless of whether or not the leader fought amongst them. If Jack dies what happens to the military? Is Jack the head of the military or is there a general that handles that stuff? Why would an army want to fight for a king that has proven that he doesn't ACTUALLY care about their lives or the lives of their families? Because he gave a couple speeches? 

Why don't lords have their own guards? Surely the various noble houses in this world have their own bannermen and the like? Swearing fealty doesn't mean Jack just owns everything about them.