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.

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.

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.).