Also to put things in perspective, Anders Behring Breivik, the Norwegian terrorist who killed 69 teenagers last summer, will get a maximum of 21 years in prison. This is the highest sentence possible under Norwegian law.
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.