This pressure has two other implications, laid out in War and Peace and War:
1. Reflux expansion: a society facing an intractable enemy will conquer or merge with more tractable neighbors. When a durable frontier forms, the societies on both sides will become organized.
2. A frontier is a potential space: sometimes, an enemy of both sides moves into the frontier and destroys or subdues both sides. Turchin's textbook example of this is the Breton-French fighting which made it possible for the Vikings to settle Normandy (and a few cities in southern Brittany).
For China, Turchin talks of how the constant confrontations with unconquerable nomads kept the civilization alive much longer than it would've otherwise endured. Toynbee thought that China should've fallen apart for good during the Three Kingdoms -- that the Han Dynasty was its universal state. China eventually stagnated, but has been the same culture since the 200s BC; the only other civilization to endure like that was ancient Egypt.
I'd really strongly advise that you look up War and Peace and War. It sounds like you understand the book's premises well, and could get a lot out of it.
For China, Turchin talks of how the constant confrontations with unconquerable nomads kept the civilization alive much longer than it would've otherwise endured. Toynbee thought that China should've fallen apart for good during the Three Kingdoms -- that the Han Dynasty was its universal state. China eventually stagnated, but has been the same culture since the 200s BC; the only other civilization to endure like that was ancient Egypt.
I'd really strongly advise that you look up War and Peace and War. It sounds like you understand the book's premises well, and could get a lot out of it.