Why not allow for additional retention as a paid add-on to the existing service? I can understand why Unlimited storage combined with Unlimited retention don't

Why not allow for additional retention as a paid add-on to the existing service? I can understand why Unlimited storage combined with Unlimited retention don't work well with a fixed monthly pricing model. But couldn't you keep the Unlimited storage and marry it to a per-GB cost structure for retained versions beyond 30-days? Then it would be up to advanced users to opt-in to this paid extra so that they can take advantage as they see fit (similar to Dropbox and Extended File History). Ideally all this would fit into the standard service... but if CrashPlan couldn't make that work economically then perhaps a different approach would be best?