They absolutely should be criminally liable for risking the lives of everyone on board because they won't take a different laptop, or none, with them.

If the search is only 5% effective then the deterrent needs to be something you wouldn't risk a 5% chance of happening, year in prison, or a fine of 2 times your average annual income seems about right?

You mean something like dying a fiery death because your laptop caught fire on a plane?

That's a lot rarer than a 5% chance. If 5% of these laptops exploded every few hours, there wouldn't be many of them left by now.

If the actual negative effects are a lot rarer than 5%, then the expected punishment should probably also be lighter.

I don't think OP was implying that. There are Macbooks that look identical in appearance to the affected devices but are perfectly fine.