> I guess if the price of BTC goes up, then it makes it more "worth it" to throw more energy at mounting a double-spend attack

Close, but not correct. As the price of BTC goes up, so does the expected reward for mining any given block, making it profitable to spend more electricity in pursuit of the rewards. This results in a net increase in electricity usage as everyone spends more in an attempt to capture the rising value of mining a block successfully.

In practice each miner is incentivized to spend an equal amount in electricity per block (at local rates) as the expected reward per block will net them. This results in a 51% attack being inordinately expensive, which was the point of the original design.