" it’s CO2 impact per unit will go down and down"
No it won't. That' s the principle of PoW (proof of work). The network is secured by burning in energy a fraction of the funds it transfers. The miner are either rewarded by inflation (new bitcoins) or transaction fees (increasing as the upper limit of total bitcoins is reached).
This way, it would cost more for an attacker to "mine" an alternative thurth (blockchain) than the gains.
This fraction depends of the consensus : speed of the network, size of blocks, etc, but is rather stable. It is estimated to 1% of the funds :
https://arxiv.org/abs/2004.04605
The total funds transfered yearly by the banking system is estimated to a few quadrillon $ (10^15).
1% of this is around 10 000 billions $. At 5 cts per kWh that's 200 000 billions kWh = 200 000 TWh. That's ten times the world electricity production.
This thing is ecologically & physically not sustainable. We should take it down.
