I get that lots could be irritated by it, but it's not exactly an illegal action by Accor to alter the terms and benefits associated with their subscriber discount program. They gave the required notice of changes to terms, and I note the ALL Plus Explorer FAQs (currently offline as they update the site) have already flagged that 2026 status tiers will be determined based on 2025 qualification.
What I do agree with is that it would cause headaches for them (their support staff at least!) to mess with everyone, and so they may well leave status end dates 'as is' for those whose status was not retained in 2025, but whose membership was set-up or renewed between Jan and Sep 2025, extending status into 2026. We won't really know until January. Get your popcorn ready, I guess!