The energy usage of Bitcoin is directly related to the revenue available to BTC miners. As mining rewards decline, so does energy expenditure to mine BTC.
BTC miners make significant money from transaction fees which largely offsets future drops. Unfortunately, if rewards drop enough 51% attacks become possible so Bitcoin requires massive energy expenditure to remain viable.
The basic equation the rewards from mining using X hardware over it’s useful lifetime vs the rewards from using that hardware for a 51% attack.
You’re looking backwards not forwards. Assuming transaction fees of 1BTC / block, the next drop isn’t to 3.125 BTC it’s ~4.125 BTC and the drop after that is to 2.5625 not 1.5625. In roughly 3 drops subsidies will be less than block fees.
Not that 1BTC per transaction is anything close to a constant, maximum rewards over the last 3 years where more than 100x minimum rewards, but it does seem independent of block rewards.
With every halving, assuming price remains constant, BTC miner revenue, and with it, energy expenditure, declines, until it's 10% of current energy expenditure.
You're right that this could pose problems for Bitcoin's security, but that's unrelated to the fact that Bitcoin miners' energy expenditure as a share of BTC price will decline.
It’s nowhere near consistent enough to say 10% going forward, as block rewards per day have been anything from 1400 to 10 BTC per day over the last 3 years.
