You're missing the point - it's not about making receiving the cryptocurrency illegal. It's about making buying the cryptocurrency illegal or otherwise infeasible. The unit economics of ransomware stop making sense when more of the victims are unable or unwilling to pay. Making cryptocurrency illegal makes ransomware victims less able and willing to pay the ransom.
So malware writers will switch to different techniques to obtain ransom. For example, there was an online illicit marketplace akin to silkroad that ran years before cryptocurrencies were even invented. It used Western union money transfers, if I'm remembering correctly, and ran for years before law enforcement was able to shut it down.
Western Union transactions required someone be physically present to pick it up, so you could catch them. How do you freeze a Bitcoin wallet or find who is using it in a straightforward manner?
