That's a lot of work for little reward, I think. Most of the time the idea is that the hackers have every incentive to decrypt or else word will spread that it doesn't work even if you pay so people will stop paying.
Then, the onus would once again be on the legitimate ransomware developers to prove their ability to do business, and some sort of Ethereum based automated contract might be an ideal tool for that.
The attacks work on those who didn't have protection and lost valuable data. Just sticking a "decryption guaranteed by Ethereum contract" will likely have the same effect on them.