Actually, let's answer the question: What *does* happen if the market is smaller than Tesla thinks it is?
Well, here's what happens: Tesla sells cells to other manufacturers. It's not like they're only going to be able to go into Tesla's cars, they can sell packs to other manufacturers and make money on that as well. Or they can sell to tech companies who make non-competing devices. If other manufacturers are making cars which people like more than the Model E, then Tesla will just make as many Model Es as they can sell, and sell the other cells to other companies, thus getting money from general adoption of electric cars rather than specific adoption of only Teslas. Now, this might mean less Model Es, and it might mean that Tesla isn't a dominant car brand and doesn't live up to everyone's expectations, but it also means that Tesla will continue to be successful as a provider of something, even if the car business fails. This is diversification and business people usually love that sort of thing.
edit: not to mention, if US EV demand last year, 2013, was ~100k units, and if EV demand is growing much faster than hybrid demand grew when they were introduced, does anyone in their right mind think that worldwide EV demand won't be 500k units in 2020? That would take very slow growth to achieve that. And heck, even if it doesn't work out, Tesla could still sell batteries to hybrid cars, etc