I read that the plant is operating way below what the capacity should be, so this could be a reflection of them trying to improve on their utilization of what they have now instead of throwing money at the problem.
Not sure I understand the difference in costs between 2 lines at full capacity vs 3 lines at 66% capacity. Obviously 3 lines is an extra 50% on hardware cost, but what's the cost of time to refine and make mistakes (not to mention the lost revenue on producing more cars) on the last 33% capacity? Apparently Elon would rather get closer to 100%.