1) Ransomware authors have obvious economic incentive to decrypt, and no reason not to. This makes it a herculean task to convince the general public that they wouldn't do so.
2) By the time your data is encrypted, you'll be researching your specific ransomware strain and will find out if it's legit or not. Googling the onion address is an obvious choice and something the ransomware author can just tell you to do.
3) Most people will need someone more technical to arrange the bitcoin payment anyway, these people will verify if the ransomware seems to be legit or not.
4) People don't magically get smarter, phishing still works if you pass the spam filters.
5) Winlockers were immensely lucrative even before they started using crypto.
6) Unless you're going to run your fake-ransomware campaign at an immense scale you'll never drown out the real, working ransomware.
And then in the end, what the was your goal anyway? Good job, now you've deleted millions of peoples data on a retarded mission to "stop ransomware". But hey, at least you stopped those evil russians!!!
There are precisely zero good arguments for preventing people from decrypting their data.