If all cryptocurrency was banned from use in marketplaces and exchanges ransomware would be less common. I know it will never happen since politicians barely know how to use an iphone, but eh.
So, i just checked.
I june 2019 the total supply of Bitcoin was 17,754,100, worth about 176 billion USD with today's rate. Human trafficking alone is a market generating 160 billion USD per year for traffickers, according to the ILO report from 2014. So, all of the BTC in the world would barely be enough for a year of human trafficking, let alone other criminal activities. Which is my point, currency is not the problem, bitcoins are just easier to use. I can guarantee you that if BTC didn't exist, people would have to pay ransom in form of mailing gold to a p.o. box in Ukraine. Bitcoin is, if anything, a convenience for the victims.
Also, according to the Business Insider, "ransomware generates over $25 million in revenue for hackers each year." This is a tiny niche, worth less than some companies spend on coffee.
As a bonus fact, according to the same report, only about half of ransoms are paid in cryptocurrencies. The rest is your USD/EUR, piped through countries where banking confidentiality is treated seriously.