If there was any chance that the victim didn't believe that you would return all the data, then they would not pay.
I'm not so sure of that. As a victim, you are aware that you have about 0% chance of getting your data back if you don't pay the ransom (unless you had a good backup setup somewhere). So you pay because you believe your data is important enough to justify the risk. This is similar (though nowhere near the same level) as someone demanding a random because they kidnapped your child. You already are well-aware that they are not trustworthy, but you really don't see an alternative.
Now, I do agree with your other point. If news was out that people paying the random did not get their data back, then I'm sure a smaller percentage of victims would be paying. But some would still pay, because their data is important enough to them that they hold on to that small hope that they might get it back.