By this logic the Great Firewall of China is a massive failure because some guy, somewhere, was able to access illegal content from China. But that's not the case. It works quite well to control public opinion and it doesn't have to be 100% effective at blocking content to do so.
The same applies to disincentives to ransoms. Of course some people will still pay ransoms. But why let criminals have a monopoly on the market? The total ransom someone can pay is fixed by the market (basically how much your child is worth to you and how much money you even have, otherwise there'd be no ceiling on ransoms). Since the government is in on the action, the revenue from ransoms is split between fines/jail time/criminals. If the government's actions are decided ahead of time, and cannot be changed based on any single ransom attempt, then kidnappers will know that their potential income from ransoms will be severely cut, thereby disincentivizing ransoms by some degree. The degree to which it is disincentivized depends on how good of a competitor the government is in the market (enforcement/detection of ransom paying events).
> Also I'm not sure how the fine would even be decided to be effective? 3 years in prison? Would absolutely gladly trade it for the life of my child. Probably any amount of time in prison would be spent gladly in order to save your child. And once we start throwing ideas like life imprisonment(because really, can't go much higher than that) it would be struck down as not proportionate to the crime.
These are implementation details. If there is no ceiling to what you will pay/rob/steal/do then don't let the kidnappers out there know! But also, you'd be a massive liability to society assuming other people don't care about your kid as much as you do, since criminals can leverage you to a 100% degree. If they know this about you, they don't even have to estimate your net worth. They can get you to go rob a bank instead!