It would also mean that Apple built a facility that will be outdated in 1-3 years. They'd end up falling behind and have to go back to TSMC or someone else to supply components or invest billions every year to build out improved fabrication facilities.

Certain kinds of vertical integration can last, but when a key part is a moving target (fab capabilities) or commodified then they are inherently time limited assuming any kind of competition (and there is plenty of competition in the PC and phone markets still).