I think the biggest thing you are missing is that Softbank gets a guaranteed return on IPO. If their shares are < agreed_return Uber issues them shares to make

I think the biggest thing you are missing is that Softbank gets a guaranteed return on IPO. If their shares are < agreed_return Uber issues them shares to make up the difference. But after the IPO they are on their own. So if the price drops or they hold a huge position it can be a problem.