Now let's say you're in the middle of a murder case, and it is about to be thrown away because of lack of evidence. So they ask you to provide the key to your encrypted drive, but you've forgotten the key... So now do you go to jail on a life in prison sentence because of it? Do they let you go? Do they keep you in prison while they try to decrypt it which might take millions of years?
I can always say "I forgot, this whole ordeal has affected me psychologically and I can't seem to recall the key because of it". Then what happens? I go to jail for 4 months? Which might be a total injustice since there's a possibility it's actually true. Or maybe it's a loophole for a crook because he might have had to serve life in prison, and now he's only doing 4 months thanks to encryption.
A judge can easily send you to jail if there's proof you did something. The same way your girlfriend can easily send you packing because she has proof you cheated. The question is, can you send your girlfriend packing for cheating if you have no proof? If you have no proof then you're sending her packing because of an idea on your head (not real fact and you might be wrong), which is hardly fair and will probably be looked at with contempt and frowned upon by peers (read the rest of the world). Now can the state send you to jail because it thinks that you have child porn on your hard drive, without ever proving you have it?
What happens then when the state has the power to lock you up without proof?