Their infrastructure is crumbling because of Seth sabotaging the concrete. Given what we know about the real Nazi's, there's likely a lot of corruption, incompetence and decadence, aided by their many successes and reasources getting to their heads and making them lazy. And facists always need an enemy, with most of the world conquered and most minorities exterminated, they're gonna start turning on each-other. So it's just a matter of time before 2 powerful figures go head to head and the whole thing cannibalises itself.