Once we get multiple 200+ mi affordable EVs on the market (~2017), and people test drive them, get to know their low operating/maintenance costs, I think they will blow past the eco crowd into the 'fun car' crowd.
If that happens, then yes it will be a big boon for electric car sales, but there's nothing on the horizon like that. I don't consider a $40-$50,000 car affordable. At best, it's the lower end of luxury car pricing. I'm really curious to see what things look like in 10 years, but the electric car pricing has a long ways to go.
And yes, I know Elon Musk claims the Model 3 will be $35,000 (which is still not in the "affordable" category - an $18,000 Honda Civic is affordable. A $33,000 BMW 3-series is not.) and available next year. However, Musk doesn't deliver anything either on time or on budget - and I'm pointing this out as a fan who has been following his work for over decade, long before most people had a clue who he was. He does cool stuff, but you have to take his performance and price figures with a big grain of salt.
The Model S was supposed hit the market in 2010 at $60,000. It came in at $70,000 in 2012. The Model X was slated for 2013 first delivery. It actually happened two months ago.
Likewise, his SpaceX Falcon 9 rocket was supposed to cost $27 million and start launching payloads in 2008. It debuted at $54 million with its first test launch in 2010, and first customer launch 2012.
The Prius was a failure IMO because it sucks to drive, and the value proposition is not there unless you log a LOT of miles.
The Prius has been pretty far from a failure - some of their competitor's best models don't even sell as well as the Prius. 2014 sales were 207,000 Priuses, versus 110,000 for the Mazda 3. Nissan's Altima outsold it at 335,000 units, but their second best model, the Sentra, only garnered 183,000 sales. Despite not being fun to drive, it's been quite popular. It's carbon emissions are almost as low as the Model S, too (based Tesla's own figures).
And the payback time has actually gotten pretty reasonable, depending on what car you're comparing too, even with our current low fuel prices. At $24,200 starting price, it's down to a mere $1130 price premium over the Camry, which it's closer to in size than the $17,230 Corolla. While that works out to a 225,000 mile payback compared to the Corolla, it's only a 29,000 mile payback compared to the Camry. There are Prius' out there that have exceeded 225,000 miles, though:
http://priuschat.com/threads/300-000-mile-club.137360/