This question doesnt have the kind of answer youre looking for. In the case of languages like Manx, it was replaced with a bit of a clean break. Anglo-norman did not just stop being spoken like this. What happened instead is that Early Middle English became more like Anglo-Norman, and AN became more like EME. You didnt have a single last speaker because the point that it stopped being one thing and started being another isnt so distinct. Both were constantly in a state of change headed toward convergence, as is typical of this sort of stable multi-language situation.

So really the question doesnt have an answer as it was asked. It was a fade, not a cut, and one involving convergence in the lead-up.

Im on mobile but ive answered a similar question to this somewhat recently which should probably be easy to dig up in my comment history. If i can link to it from mobile Ill do that.