While the Difficulty is bound, the theoretical energy is not as it can not be consumed. Could it not come to a scenario where we can not change the difficulty anymore but the hashrate still continues to grow?
is this limit realistically reachable? Or not? We probably don't need to worry about it. Satoshi is not worried. ;)
Also, there can be an easy soft fork when there is something needed to fix at some point. For now it is all good.
4. Risk of bound vs. Unbound