I think there will still be three pack sizes for the Gen III. I think Tesla will make a "$30k" version but much like the 40kWh S it won't sell much. But having a low entry point for something like a Gen III, or car for everybody, you need that entry point.
When people come into a store, look around, figure out how much operating costs will drop. They can be shown that they can afford $10k or more than an ICE base price. Get moved up to the second tier battery and be much happier. I think Tesla is seeing plenty of demand, without a low price point entry point with the Model S. So they axed the 40kWh pack.
And remember the average new car purchase price is well into the $30k range right now. Considering fuel savings you can easily tack on $10k to the purchase price. I don't think Tesla will have any problems selling $40k and $50k Gen III vehicles to normal middle class Americans. They will just have to get them into the door, and educate them about TCO.
I also think that Gen III will benefit from a much better residual value estimate, with the LEAF and Model S being sold so much. And I also expect a battery reuse ecosystem to be starting around that time, giving more demand for a used battery, raising the residual value even more. All of this makes a higher purchase price easier and easier to swallow. Not to mention makes leasing the cars out at very low rates possible.