The regulations alone won't make this easy though. If it's a "savings account," then you'll be subject to many banking regulations, and FDIC requirements, and stress testing that comes with DFAST regulations after the financial crisis. You can take customer money and just go out and do anything, which in this case would be setup a factory. The regulators won't allow it, I guarantee it.
You can also structure it not as a bank account, but an investment vehicle, and now you get into securities regulations, and that's a whole different ball game too. The recent DOL ruling could add another layer of complexity depending on how their representatives are looked at in the laws.
Look, in general, I think this is a really good thought exercise and there is most definitely a portion of the customer base that would benefit from this. I just don't think a company like Tesla is going to shift even 1% of their focus at this point to taking deposits from customers.
20 years from now, whether they paid 5% for short term borrowing or 0% from customers won't have made a difference. No one looks back and says, "Gee, Coca Cola would have only been a good company if they paid a little less in interest back in 1965. If only..."
Again - good thinking, but they're not at the point yet to optimize interest costs this much.
