I think you're underestimating the issue with attackers, if the world ran on bitcoin, attacking it becomes much more attractive, so use must indeed use more energy. It must use enough to make attacking bitcoin very unattractive.
No, sorry, that's not how Bitcoin works either. If the whole world used Bitcoin and the market cap of Bitcoin was over a trillion dollars, a 51% attacker could still only double-spend its own transactions. As long a public key crypto is secure, you cannot just re-write arbitrary Bitcoin balances into your own wallet even if you had 100% of the mining power. Therefore the incentive to attack Bitcoin is only as big as your own Bitcoin holdings, not the value of the entire market cap.
>> Therefore the incentive to attack Bitcoin is only as big as your own Bitcoin holdings, not the value of the entire market cap.
That's not quite true though is it? They could certainly (for instance) block transactions and generally hold the whole thing to ransom.
You're right, it doesn't have the financial incentive that you could (for instance) steal coin, but you could potentially block all transactions that don't have a considerable fee, for you, or just break stuff. Motives to do that will become more pressing the higher value the network is to nation states.
