That isn't quite right. The 21 year sentence can be extended to keep him in prison for life [0].
He is not going to be walking out of jail free 21 years from now.
He is not going to be walking out of jail free 21 years from now.
Extending the prison term seems like the right way to go and keep the issues of punishment and safekeeping separate.
Don't know if you're from Norway, but Norwegian criminal law has a distinction between 21 years, which is the maximum sentence permitted by law, and "forvaring", which is explicitly defined as additional, indefinite custody with the express purpose of protecting society from violent criminals which have a very high risk of doing repeat offenses.