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For reasons discussed in the sub-forum three years ago (https://forum.paradoxplaza.com/forum/index.php?threads/when-the-world-stopped-making-sense-a-480-migrational-period-mod.772971/page-65#post-17399105) I do not endorse this implementation of the shift at all (as little as I came to believe in the earlier implementation of melting-pots) and believe it should be rediscussed thoroughly.
That said, based upon what we have, the question might rather be, is this better than nothing? I would argue no, because firstly the Romano-British disappear into thin air with this, definitely breaking the existing melting-pots. The disappearance of the Romano-British is an interesting subject, but the way it works in game is Romano-British -> Briton. Secondly for Breton, it doesn't correspond with the setup. Thirdly it conflicts with language ≠ culture in game, in my view.
Note that the years in province history and years and MTTH in events are open to discussion.