We are in the context of already having to pay. You are at their mercy no matter what, so the only value of any interaction with them is based on hoping they have incentive to maintain their promises to protect their reputation etc.

It's not a good situation to be in, but still, try to make the best of it.

If the ransomware operator believes that breaking their word might make it harder to get money out of future victims, they'll keep their word.

They might not believe that, but if you're at the point where you're paying anyway, you might as well try to get that commitment from them.