Why? There are many Proof of Stake coins out there too, for instance Dash and NEO (and many more coming).

I mean let me put it this way, how can you even claim that a blockchain requires more energy than a Visa transaction? Isn't the price of a transaction an indicator of the amount of energy needed for it, for ordinal comparison?

For instance, if Visa charges $0.3 per transaction, and another network (Whether it's cryptocurrencies or Gnomes carrying gold from you to the other person) charges $0.2 per tx, then as long as the two compete freely, you can say that the energy required by the latter is lower than the energy required by the former.

Keep in mind, I said 'compete' and 'freely'. Visa may have a higher profit margin because the alternate payment system isn't popular enough yet, so Visa's power expenditure could be much lower than reflected by their tx fee.

A unit of account from my understanding simply means that other people are willing to price their goods/services in your currency. This is a by-product of people participating and using the token, and it being stable enough. So once there is a "stablecoin" that gains traction, this will surely follow.

To solve proof-of-work consuming too much electricity, Ethereum is upgrading to proof-of-stake which is on the roadmap and consumes a negligible amount of power.