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.