> First, the skateboard and the upper body could be built separately. Maybe not at the same place (a handful of skateboards will fit perfectly in a shipping container for completion thousands of miles away).
I hate to be critical, but this really reminds me of a 1950s "visions of the future" piece more than a practical design choice.
For example, the skateboard is not a new concept [1], and there are reasons you don't see it. Modular design really only shines for low-volume production (or a large combinatorial space). When you are doing 10s of millions of units per year, as the auto industry does, modularity often gets thrown out in deference to volume/cost.
Also, distributed manufacturing is a huge pain, why would you ever want that? See Boeing's experience with the 737 supply chain. Just because you can doesn't mean you should.