You listed three factors. The first two, ceteris paribus, increase buyer bargaining power; the last does not. That doesn’t change because in this situation the last overpowered the first two.

By analogy, shape, size and camber contribute to a wing’s stall speed. If a wing has the wrong camber, that doesn’t negate that all three are components. (And that wing, in turn, doesn’t disprove flight.)

> Why would you outsource building a plane wing? They are not standardized but different for each plane

This is true for a lot of specialised manufacturing, e.g. TSMC and Foxconn. Varied lines is not an argument against specialisation.

> they are huge so you can’t exactly build retoolable conveyer belt production line

The first moveable assembly line built ships at the Venetian Arsenale. Size is not an argument against specialisation.

> only reason to do this is to safe on labor costs and because you don’t want the hassle of doing it yourself

Not true for Spirit. In that case, it was labor complexity and capital costs.

> primitive MBA thinking

Shanahan, Spirit’s CEO, was an engineer before MBA.