Well predictability is known false, but I didn’t imply it? Or at least I tried to avoid the implication by saying what I said. If you bear that in mind, will you be able to grok the rest of the sentence?
According to determinism there is, in principle, a unique path from one moment to the next that a perfect physics would, in principle if not in practice, predict. We say ‘determined’ to mean ‘predicted in principle by the physics being true about the world even though prediction in practice is proved impossible’. Don’t we? Or how is a chaotic system deterministic yet unpredictable?
Then, if, by hypothesis, free will is the intervention of uncaused causes (insights, decisions etc) into an otherwise deterministic sequence of events or global states, then it’s plausible and interesting to conceive of the free will ceasing to occur, leaving the history of events exactly as determined (predicted in principle) uniquely by the perfect physics.
That such a cessation might follow from a belief in determinism would then suggest further questions with a flavour of paradox. Hence my suggested tweak of the OP.