Bitcoin mining difficulty is automatically adjusted based on the capacity of the network. If the cost outstrips the gains, then either people are content to los

Bitcoin mining difficulty is automatically adjusted based on the capacity of the network. If the cost outstrips the gains, then either people are content to lose money on the network, or people will leave until the difficulty becomes low enough to make mining profitable again. You're probably correct that you'd only need $300K to purchase enough hardware to double-spend, but any profit you could make off double-spending would be offset by the cost of electricity. Also, an alternative compromised blockchain would be really obvious, so you'd only have a limited window in which to scam people. Maybe you could do something if you had a botnet (and thus didn't need to pay for the hardware or electricity), but I can't help thinking that you could probably be doing far more profitable things with that amount of hardware.