Bitcoin mining is actually harming the environment which is why they aren't entirely wrong in this case. This is why energy-efficient miners have become popular in recent times.
There is no such thing happening.
A car becomes more efficient when it drives on less gas so all the cars in a country will consume less gas.
With cars, no matter how cheap gas gets you won't be driving 10 cars at the same time because gas is 10 times cheaper.
Miners with a better J/TH efficiency will not make the network consume less energy, they will just increase the hashrate for the same amount of energy spent, but if the reward goes up 100 times, miners will add 100 times more miners.
The energy consumption is dictated by only one thing, the price of bitcoin, that doubles, so will the hash rate once enough miners are purchased, the price goes to 10$, the whole network will consume less energy than a merry-go-round.