[cesarb]

> So in theory, it's possible to just trade bitcoins off-chain (either on another blockchain or just using boring centralized tech) without necessarily incurring a corresponding increase in mining-related costs, right?

The mining-related costs are mostly unrelated to whether the bitcoins are traded on-chain or off-chain. The mining-related costs are more directly related to the price of bitcoin, or rather, how much a miner can earn with the block rewards and transaction fees. By trading bitcoin, even off-chain, you increase the demand for bitcoin, and therefore indirectly increase its price; the only difference is that by trading off-chain, the miners don't receive the transaction fees (which are currently less than 2% of the block rewards).