According to https://digiconomist.net/bitcoin-energy-consumption bitcoin is on track to consume 19 TWh this year. That puts it at .1% of the global power consumption already
NASDAQ does 10 million trades a day, roughly. So if bitcoin scales to just handle NASDAQ alone it would consume ~850 TWh/year, which is 3.5% of the total world's electricity generation (to say nothing of Visa's 150 million transactions a day)
The reason bitcoin isn't currently an enormous threat is because it's basically barely used by comparison. 300,000 trades/day is a tiny number in the world of financial transactions.
Edit: mining effort is dominated by difficulty anyway, not throughput.
The key question raised in the parent comment, which your own riposte artfully dodges, is once again - who stands to benefit from this N% increase in global energy usage?
As far as a justification goes, I think that if BTC reaches 2 trillion, it will have gone mainstream. Meaning either existing transactions should become easier, or people find it a good store of value.
This isn't just theoretical. The marijuana industry faces immense hurdles dealing with the traditional financial system, because marijuana is technically still illegal at the federal level. This makes it a nightmare to do any of the normal banking operations we take for granted. Bitcoin sidesteps this problem while also letting you be legal: As long as you declare your taxes properly, Uncle Sam probably won't come knocking.
That's just one example of an industry that benefits. There are many more.
[1] https://en.wikipedia.org/wiki/World_energy_consumption [2] https://www.thedailybeast.com/the-insanely-profitable-federa...
Is less than 1% additional energy usage worth a 2 trillion cap? Well, I'm sure people can find a way to argue that it's not, but the point is that bitcoin isn't this enormous threat to energy usage that people try to argue it is.