It's more expensive to manufacture the same amount of compute power using older processes. That makes old silicon not competitive. From the viewpoint of the consumer, indeed, price per compute power is relatively constant, but from the viewpoint of a silicon manufacturer, newer processes are cheaper per compute power.
This doesn’t answer the parent point. Compute is more expensive with older tech, so what? They’ll just pay more. Consider North Korea. Their ICBM technology is pretty bad compared to the state of an art, however it is still great security concern.
They won't sell, and not enough to fund competing research. I'm pretty sure the security concern isn't directly related to defense, but to the security of US industries and economy.
