The problem is that no one can mass produce batteries for cars at this scale. As of now the projected capacity of the Gigafactory is enough batteries for 500k cars / year. By 2020 it will be 1.5M cars / year. This is more batteries than the whole world produced in 2013.
By 2020 Tesla is gonna have a lot of experience how to do this efficiently so the cost will be much lower. They are gonna have a new gigafactory in the EU. Lithium mines can't produce enough lithium at this scale so Tesla already makes deals with them to ramp up the production.
BMW makes 2.5 million cars a year. So what's the plan exactly? Just buy the batteries? From whom? When every other large car manufacturer wants to buy batteries and there is not enough on the market the price will go up.
Every time I read something like Volkswagen|Mercedes|Audi|whatever plans to sell x ev cars by 2020 I never see any mention about who is going to make the batteries.
I think the large car manufacturers are already behind, they should start working on their battery factories right now.