I have a thesis that every Model S that makes it into the wild will create some number of new buyers that weren't there before.
Think about it, Roadster owners are always saying that they get stopped all the time and people ask about the car. I mean, you are a walking, talking, driving advertisement for Tesla. How many of those people will go home and look up the car? How many of your friends and family will ride in it a couple times, and be impressed, and add it to their shopping list? They won't all buy it, but of the, let's say 25 people that you rub elbows with in the car (and I don't know if that is a reasonable number or not), what if one of them does? If every Model S sold can create 1 new buyer, then 20,000 units a year is sustainable. But what if it creates 2? Well, then maybe we've just created the market for the Model X. What if it creates more? Then we have a serious lack of supply problem.
Yeah, right now if you ask people if they would consider buying an EV, they will likely say no, but I'm sure if you asked people in the early days of flight fly in an airplane, they would have said no too. But, once you know someone that has an EV, and realize that they've never been trapped on the side of the road, and experience the performance, and see for yourself how good they can look, etc. etc...then maybe it at least makes it onto your list.
I just don't know what the number is. How many new potential customers will each Model S create? I feel like it is a number greater than 1, but less than 5. This is why I'm not concerned with the pool of buyers running dry in 2014. Even given that the Model X will double the number of customers we need to find.