Let's say for sake of it and they paid ransom of $10M. What the decryption process does is damage data severely. But that is not the only thing. One thing they do is load so much spyware on your systems, you will never get rid of it except through wiping your hard drives, restoring both the systems and data and then installing antivirus and other software and investing more into hi-tech security systems. The most high profile example of ransomware attack is Baltimore City Government. BUT they paid over 17 million dollars recovering data when the ransom got dropped to $52,000. Prior examples had proven that more often than not, encrypted files remain encrypted even after paying the ransom. And the "encryption key" they send you often contains even more severe malware.