It's not a "waste" if the result is a decentralized, trustless, robust digital currency.
And the global warming argument is a many times debunked hoax - if anyone has better use of that electricity, they are welcome to use it, which will make mining cryptocurrency unprofitable. Except in the other cases, the benefits go to the single entity that owns the business, whereas with cryptocurrency every participant benefits from the stronger network. Global warming is caused by using fossil fuels to make electricity, mining crypto doesn't require fossil fuels. There are other ways to make electricity, let's focus on them.
> if anyone has better use of that electricity, they are welcome to use it, which will make mining cryptocurrency unprofitable
I'm sorry, but you got this backwards. If bitcoin becomes a relevant part of the world economy, then it HAS to use a relevant part of the world energy, because consuming energy for PoW is the only limiting factor against a 51% attack. So, either Bitcoin is irrelevant - and thus its power usage is a literal waste - or it's relevant, and then its power usage has to become relevant in terms of global warming.
From the point of view of the miner, they have a supply of electricity at a certain price and can decide what to do with it. They might run their TV, computer, heating, industrial machinery, datacenter, or a bitcoin miner. They have to compute what profits can each of the options give them. If running bitcoin miners at that electricity cost and at that profit they can get from selling the bitcoins is more profitable than the others, then it's logical to do that. If on the other hand the price of bitcoin goes down, or the price of electricity goes up, or people start paying more for server hosting, then the decision changes. The fact that miners are now mining, means it's currently most rational to do that, given what society (users, participants) are willing to pay for the services.
If bitcoin is the dominant currency, another option exists:
51% attack, double-spend to get free electricity.
This is useful even when it makes an apparent loss by damaging trust in the currency: If you are, say, the USA president and you’re at war with Iraq, and Iraq uses bitcoin, you can outspend on energy until they surrender.
Unless the whole world uses bitcoin, but then the first few nations individually face the same problem, regardless of who else actually uses bitcoin — gotta keep the USA and China happy at the same time! (The EU isn’t integrated enough to do that sort of thing yet, but is a similar sized group).
Right now, 51% needs the cost of one very large power station from when you start until when you win — large nations, the sort with global ambitions, can spare a lot more. In the previous example, that’s close to all the power Iraq produces, but 5.4% of the USA’s output.
Then there’s the fact that most countries like being in charge of their own currencies as the ability to create or destroy units is a useful economic lever.
> The fact that miners are now mining, means it's currently most rational to do that, given what society (users, participants) are willing to pay for the services.
Or it’s speculation, like so many other things before and probably yet to come.
There is no such thing as speculation by mining - if they wanted to speculate but didn't profit from the mining itself, it would be better for them to speculate by buying - instead of giving dollars to the power company, they'd just give them to other people on the exchange. Basically the only reasons for miners to "speculate" are long term contracts like lease contracts for warehouses. And those may or may not be long term.
And 51% attack has the designed side effect that all other people stop using the currency - they may keep following the previous fork without the double-spend - as shown by ETH and ETC. You'd also need some way of forcing all participant to stay with the now corrupted currency. Otherwise the attacker who performs 51% attack will gain the ownership of a network that immediately becomes completely worthless.
> And 51% attack has the designed side effect that all other people stop using the currency
That’s my point. That is literally the point. That is why it is a bad thing and why no sane nation would ever allow it to become their main currency. It is an attack surface. It is a vulnerability to your economy.
I see no benefit in a trustless currency: If I have no trust in a currency, why should I trust the goods/services?
I see no benefit in a decentralised currency: gold is one (anyone can mine it) and there’s a reason we moved away from it.
I don’t buy that bitcoin is either decentralised (they who control the algorithm steering committee control the currency); trustless (why should I trust irreversible transactions? Why should I trust those who wrote my wallet? Why should I trust those that wrote my mining app?); nor robust (the domain of money is law, not logic, so always subject to government interference; the price is currently highly volatile; and apps always have bugs yet to be discovered).
Trustless currency means exactly the opposite of what you think. It means you don't need to trust anyone, because everything can be proven and verified mathematically without any authoritative figure that you'd have to trust. That's why you can trust the system itself.
I think you have fundamentally misunderstood my point.
Let’s say I buy a widget. How can I trust that the widget will arrive? That it will do what widgets do? That it will not break? None of these are payment issues, but they might call for a refund. How can I get a refund? How can the refund system be resistant to abuse?
The answers we currently have are “the law”. If the law functions, I don’t need a trustless currency.
And the global warming argument is a many times debunked hoax - if anyone has better use of that electricity, they are welcome to use it, which will make mining cryptocurrency unprofitable. Except in the other cases, the benefits go to the single entity that owns the business, whereas with cryptocurrency every participant benefits from the stronger network. Global warming is caused by using fossil fuels to make electricity, mining crypto doesn't require fossil fuels. There are other ways to make electricity, let's focus on them.