I agree in most part with you but Mannings neck is stronger than it was before the surgery. The nerve issue is separate from the fusion. Manning is arguably g.o.a.t. at the quarterback position.
My only issue with him would be the exit strategy when he is done. The colt quarterbacks behind him all suck because he focuses everything into himself to be ready. After watching his back ups do so poorly for so long I don't believe you can transition from him to someone else.