>> Buy inferior battery packs at a higher cost from a different supplier? Try to replicate Tesla's battery technology and then convince your board to build a gigafactory so you can produce them at the required volume and cost?
Why not another option - battery manufacturer notices that if they build an equiv. battery they can sell to every car manufacturer in the world. With the higher volume they'll be able to do it for less than Tesla if they are only building for their own cars.