The court is indeed not saying that very long detentions violate art. 3 per se. It recognises that punishment--for example--may justify very long detentions. However, in paragraphs 111-118, it refers to a large body of law, policy and internationally accepted recommendations in support of the notion that rehabilitation etc. MUST be--and, in practice, is--considered in justifying prison sentences as being humane and non-degrading, even for "life" sentences, indeed even for people who've committed crimes as heinous as genocide. This is what enables the argument about the balance of legitimate penological reasons shifting with time and closes the door to the view that punishment alone--with no consideration for other reasons--can be used to justify
indefinite imprisonment. The "punishment" portion of the pie may not be infinitely large.
Because the court's view is that there must exist a balance between all the legitimate penological reasons for imprisonment, and because its view is that this balance may change over the course of an individual's imprisonment, it concludes that, "... in the context of a life sentence, Article 3 must be interpreted as requiring reducibility of the sentence, in the sense of a review which allows the domestic authorities to consider whether any changes in the life prisoner are so significant, and such progress towards rehabilitation has been made in the course of the sentence, as to mean that continued detention can no longer be justified on legitimate penological grounds."
At least, that's my reading. We may be talking past one another
It's up to the state to decide how to safeguard that reducibility or how to implement a system for reviewing whether or not a person's sentence can and should be reduced. It's also up to the state to decide when such a review can take place, and the court only notes that there is support elsewhere for conducting such reviews no later than 25 years into a sentence.
I'd like to point out, btw, that the court in this case does not act as if it--and the UK--is legally bound by the German and Italian courts, which was Dread's misdirected anti-federalist and anti-EU charge. It recognises the correctness of the constitutional courts' reasoning in relevant cases, but it does not say that the correctness of the reasoning stems exclusively from the fact that it was used by those courts. Nor does the court base its decision exclusively on the practices of the contracting states, or even only on the practices of the states it likes, which was Loki's allegation. The notion that an irreducible life sentence may be a violation of art. 3 is not a new one for the court (see the references to the Kafkaris v. Cyprus case among others) and it is, for this court, founded on substantive arguments. The references to the existing and emerging international consensus on sentencing are used to support the ruling but do not suffice to justify the ruling all on their own.