It requires great energy for etherium and bitcoin to be able to continue using the Proof Of Work (POW) which they both use. Unlike Bitcoin, Etherium is looking forward to switching to Proof of Stake (POS) to reduce energy consumption. This switch to Proof Of stake was constructed to enable blockchain be more efficient and reduce transaction cost.
It compares the energy use for bitcoin as switching from same energy consumed by a country is currently being consumed by a village. Which makes him think that Elon Musk’s remarks are understandable. He thinks that if bitcoin keep using it’s current technology without switching to proof of stake or a hybrid of the two mechanisms, there is a big risk it will get left behind.