> With larger vehicles (e.g. Starship) the JWST mirror could have been built in one piece, which is much simpler

Not just the complexity of design, but also cost and complexity of cryo-vacuum testing hundreds of deployment mechanisms any one of whose failure critcially endangers the project. The mirror could also be conventionally manufactured versus requiring gold-plated beryllium [1].

[1] https://en.wikipedia.org/wiki/Optical_Telescope_Element