How? If it isn't deterring others, then the only possible way that statement holds is if you assume he himself would carry out further attacks, which may be true if he hadn't be caught, or had been given no punishment. But that isn't the case.

At a certain point your adding years to the sentence, not because it helps the public, or aids in rehabilitating, but simply because it makes you feel good about "doing the right thing."

But often the end result is we throw what are essentially kids in prison only for them to become better connected and a more skilled criminal who see's no way to become a part of society because he's been in prison for 2 decades.