Replacing permabans with the permanent prisoner island is more humane than what we have right now without interfering with interests of the many.
You're either implying that permanently banning players is inhumane (it isn't) or that (more plainly) it's less humane compared to Prisoner's Island. And, in the likely case that it's the latter, please refer to Riot's Ask Riot post:
Reason Two: Prisoner’s island is a truly awful experience for the “prisoners” in more ways than one
If we ship unsportsmanlike players to their own queue, they’d face a swath of issues that basically make League miserable by default. The queue would face higher rates of negativity in game, sure — but the diminished pool of players would also result in terrible queue times and lopsided matchmaking. It’d be emotionally satisfying to say they deserve it, but it’s squarely against our values of putting player experience first.
Beyond that, building and operating a service for a prisoner’s island would suck Rioters and resources from efforts we truly believe in, like improving Instant Feedback, intentional feeding detection, and reworking legacy systems like Honor.
What's more humane, making someone suffer horrible matchmaking, terrible queue times, etc. - even for continuous minor offenses, or god forbid, the scant few players that break the zero tolerance policy once and never do again that you've tried to argue for before - or straight up removing them?
I don't understand how Prisoner's Island is "more humane" than permanent bans when it pretty much amounts to torture. Abstract, videogame-mechanic torture, but torture nonetheless.
Riot in that article is somewhat justifiable(and it's a big stretch anyway) only if they respond to the idea that all punishments have to be replaced with the prisoner island. They shouldn't.
Why does it matter what tier of punishment they refer to? They're addressing the concept of Prisoner's Island as a whole, not as a stand-in for any given punishment tier.