You say "there's some efficient amount of money to spend on security, and markets will aim for that amount ... will happen with negligible mining reward" I thi

You say "there's some efficient amount of money to spend on security, and markets will aim for that amount ... will happen with negligible mining reward" I think is false. See comment: https://news.ycombinator.com/item?id=8887085 https://news.ycombinator.com/item?id=8887085 I would say the upper bound on the amount of energy someone will spend on BTC is roughly the cost of (prestige of mounting a 51% attack) + (BTC market cap of one individual). If people allocate resources to BTC efficiently then one rogue person will just mount an attack and take over the network for the combination of prestige and fortune. Any person with large BTC wealth will want to have more security to protect his investment appropriately, and your security against said attacks is buying more hashing power. The fact that over time hardware becomes cheaper just means it's a constant losing battle. Other markets that allocate money to security I think can reach an efficient amount because the system they're protecting does not, as a core part of it, require you to have more processing power than your user's / people you transact with. The two security models are so drastically different I don't think a comparison can be made. Perhaps the closest comparison is password hashing where it's good practice to increase difficulty over time to slow down attacks, but with 2FA and lockouts after N attempts, you actually don't need to allocate significant resources there.