Bitcoin miners tend to find the cheapest electricity, like near hydroelectric dams. So let's assume they can get electricity for $0.05/kwh. And to keep things simple, assume that 100% of their costs are electricity (in reality there will be capital costs and other operational costs, adjust calculation to your own liking). That means they can collectively consume 18,750,000/0.05=375,000,000 kwh/h= 375 gigawatt of power before they stop making a profit.
Now compare that to global electricity use, which is on the order of 20,000 TWh/year or about 2000 gigawatt. Bitcoin miners would be using on the order of 20% of all electricity!
Except that at those consumption levels, they would start driving up electricity prices, which brings me to my point: will governments really allow everybody's electricity bill to rise significantly because it is profitable for bitcoin miners? I think not.
It seems to have a pattern leading up to and after every halving over a four year cycle (https://hackernoon.com/a-detailed-breakdown-of-bitcoins-four...), though some argue well that it doesn't (https://bitcoinist.com/is-bitcoin-price-cyclical/)