The numbers that they are providing are pretty reasonable and nobody is really disputing that removing the mining requirement and just staking your Ethereum is enormously more efficient. Even if you assume each staker is running 1 server, then 140k computers NOT running at full tilt all the time is still a tiny consumer of electricity compared to the current situation.

A lot of the debate seems to be around whether or not it's as secure or viable, or whether the existing Ethereum miners will try to stage a coup or something.

Disclaimer: I don't hold cryptocurrency and I think proof-of-work cryptocurrencies are a tragic waste.