You could show that they accessed a device or used a service immediately prior to being subject to litigation (through network logs for example). A judge would not believe the person didn't remember the password in this case. If they destroyed the keys after receiving a request for data then that would be destruction of evidence which is already punishable. If the data is held by a third party then the third party could perform a password reset.

How long does it take for cases to be brought to trial?

Could you argue the stress of going through the legal system made you forget your twenty word password?

What if the house with all the evidence burns down and all of it is unusable ashes?

What if the government goes pursuing old crimes (or divine something that points to you) and you don't remember the password?

Can you do that at the press of a button ahead of time?