There's just no other form of payment which would work for them. You can't easily go "can I have $50k worth of giftcards" and on the receiving side you can't easily validate or sell millions of them without tanking the value. Any kind of wire transfer would expose the source immediately at that scale. There's only so much money you can move through services that give you kickbacks of various kinds. What else is left?
Basically unless ransomware teams know of a new really good way of laundering money without a trail, or are happy to take a massive pay cut, that would be the end of most of their operations.
Money grows on trees, there, too.
I'll bet you dollars to donuts that if you made crypto illegal, there's still a whole lot of countries that won't give a shit and the problem will only get worse.
All these situations are nebulous and complicated, something as simple as legally banning crypto is not going to solve the problems.
It's like nobody has learned a thing from the war on drugs, my point being: you deal with the root cause of the disease (infosec in most companies and even government offices is a joke and bad people have taken notice), not playing whack-a-mole with the symptoms (crypto use) that hint towards systemic decay.
If the goal is to stop companies from paying ransom, then why not just make that illegal?
https://home.treasury.gov/policy-issues/financial-sanctions/...