Perhaps range isn't the real issue. Maybe we should think more about the availability of recharging stations and the time it takes to charge.
What is better, stopping for a ten-minute charge every 200 miles, or being able to go 400 miles but then having to spend an hour sitting at a Supercharging station? Does it being free make this long stop so much more alluring?
What happens when lower cost models come out and Tesla wants a much larger market that includes condo owners and renters? They can't install home chargers. How are they going to charge up if not at home? Will they be happy sitting for hours a week at some weak charger?
Perhaps the answer is a battery that you can quickly and cheaply charge up at most any gas station. Not free, but a lot cheaper than gas and with all the other advantages of an electric car. People will be happy enough to get the occasional free charge when they come across a Supercharger, and this will be a viable option for long trips. And they are great advertising for Tesla.
Monetizing charging will bring in more chargers at no cost to Tesla and allow for a wider customer acceptance.
Perhaps a fast-charging 200 mile battery would be lighter and cheaper than the holy grail 500 mile battery. Now you have a cost advantage to go along with more convenient charging. I would presume also that chargers that can charge a smaller battery are not as expensive to put in as those fancy Superchargers that have to force-feed giant batteries. The cheaper the charger and its installation, the more profitable they get. More chargers equals more electric cars and it just all gets better for everyone.
If capacitive batteries are what it takes to do this, and these are expensive to make, perhaps going small would at least minimize the cost some and allow for their use.
Tesla could also mass produce an appropriate turn-key, easy to install metered charger that would be attractive to gas station owners, perhaps free with a piece of the action, to get these put in everywhere.
So, maybe ubiquitous charging stations and quick-charging batteries with modest ranges are what we really need for the near future at least, to get this going. Later there will be other alternatives.