If that’s true it would only be the case if you could amortize that tooling across enough vehicles. Selling 10k-20k per year from a production line planned with a capacity of 125k per year won’t get you there.

Why?

Steel is heavy, you have to paint the frame anyway, people like cars with colors, the steel has to be maintained too (scratches etc.).

Not really. Stainless is expensive and brittle. It's a lot more work to form complex panels out of stainless, and then they're essentially scrap if there's even a tiny amount of damage. A small dent will start a crack.