The idea would be to reduce the likelihood of a payout.
The actual cost of launching an attack like this is basically nothing - initial access, etc, is largely automated and performed at scale.
The “costly” part is the hands on keyboard part, but even that can be largely automated, and even manually doesn’t take long.
This is not a company where you automate people out of job and CEO gets all the profit. Organised crime groups share profits among themselves, and the profit is by far the main motivator for all of them.
This would basically remove the prospect of million dollar payouts; it probably removes the prospect of payouts in the hundreds of thousands. Any company with the money to make those kinds of payouts is likely to have reporting requirements that make it very hard or impossible to hide.
Payments in the tens of thousands could maybe be hidden or targeted at small enough businesses that they don't have to report what happened to their money, but is it even worth it at that point? We're talking people with at least some level of technical ability; do they really want to piss off the FBI/NSA/European equivalents for tens of thousands of dollars? I sure wouldn't.
Do you think we should do similar for theft? Should it be illegal for a store assistant to hand over money to armed robbers, because theoretically if less people handed over money there might be less armed robberies?
And I disagree with what you're saying anyway. I doubt this would stop ransomeware. I think if anything this would just push ransomware to become even more cruel so that they increase the likelihood of their victims choosing to break the law over not giving the ransomware owners what they want.