I think you are making this more complex than it really is. The assumption that I am trying to make is that before options the 70D and 85D are priced so that they have the same GM. So this condition is entirely within Tesla's control as they set the price. This condition alone is sufficient for the 70D to deliver more gross profit per kWh than the 85D. Next, if options are priced the same for both versions, then it is easy to expect that more options will be sold on 85 70D than on just 70 85D. So again 70D delivers more option revenue and grow profit per kWh.
The thing we should be discussing right now is whether cells have become the dominant constraint again or is the bottleneck somewhere else, even demand. With improvements to the production line, there may be excess unit capacity relative to other constraints. If Tesla is in fact introducing a new higher density cell right now, then those cells may be in short supply for a while. And this would explain why Tesla is not offering a 100 kWh battery at this time. It can make lots of cars, but it cannot make lots of 100 kWh battery packs. So how does Tesla make the most of its limited high density battery supply, unit capacity and surplus low density battery supply? Answer: make lots of 70D units with high density batteries, make lots of P85D and 85D units with low density cells, and pack surplus low density cells into stationary storage products. As the stationary products advance, they will eventually off take all the low density cells. Meanwhile, production of high density cells ramps up. As soon as there is sufficient high density cell production, Tesla will announce a high density 85hd pack or a 100 pack. I would expect a 100 kWh pack for the Model X before it will be made available for the Model S. That is why a highhigher capacity battery is not coming for the Model.S this year. It's going to take that long to ramp up the HD cell supply. In the meantime, Tesla needs to optimize gross profit subject to a limited supply of HD cells; hence, 70D is the product to push at this moment.
And if demand were a concern, upgrading the entry version of the Model S is a great way to tap a value segment that has so far been quite overlooked. I expect solid unit sales from this segment.
I'd also point out that Tesla is also wanting to deploy charging infrastructure rapidly. This is one reason why giving a little more emphasis to unit sales at this point makes sense and why it makes sense to include Supercharger access standard on the 70D. It is important that this infrastructure is well used. Underutilized charging infrastructure looks bad on Tesla and does not encourage others EV makers to participate in this network. Additionally, I believe that the 70D will help with word of mouth and viral growth. You need lots of enthusiastic owners touting how advanced and affordable the car is becoming. This is a much better message to the next wave of customers than the standard line about how in 3 years Tesla should make an affordable car. For many fence-sitters, the Model S just became a much better value for the entry price. Unit sales matter.