The problem is that with a competitive mining market, Bitcoin energy use will rise until the cost of mining a block is equal to the reward for that block (i.e. market equilibrium). So if the cost of energy goes down but the price of Bitcoin stays constant, the amount of energy expenditure will be (approximately) equal.

Likewise, if the price of Bitcoin goes up, the amount of energy used will go up accordingly.

The reward amount will decrease over time and mitigate this, but not enough by a long shot IMO.