Thinking about it some more, there must be a break even point. Having many configurations means a higher initial development, testing and tooling effort. This breaks even only if the savings per unit accumulate to offset that overhead. So Tesla might be big enough for serious automation, but not yet big enough for actual prodyct customization on the factory floor. Pure speculation, though.

I just looked up how to add fog lights to the Subaru forester without fog lights. Turns out you need the lights and the switch, and no wiring.