An effective thing they could have done is to announce that they paid the ransom, but that the decryption did not work (even though it did).

That has the advantage of discouraging other people from paying up, and therefore reduces the incentive to create more ransomware attacks.

Hell, the government could step in and recruit people and companies to falsely claim that they were ransomware victims who paid up, but never got decryption keys and were screwed over. That could put a damper on ransomware psychology.

Thinking about it further however, this will probably lead to better "customer service" by the ransomware makers. They'll adapt their software to selectively decrypt part of your data for free, so they can offer you proof that that they can and will give your working keys once you pay up.