The ability to convict a particular case on a particular set of evidence is unknowable before trial, where the facts are decided by a jury. It would be irresponsible for a prosecutor to attempt a trial if they know stronger evidence is available.

The prosecution was able to convince a judge that there is encrypted child pornography on those drives. The judge can't force a jury to accept that (a ruling of fact), but (s)he can make a ruling of law that the defendant no longer has a fifth amendment defense to producing a decrypted version of those drives.