I think gas prices are driving EVs less and less. It's still a factor, for sure, but things like performance, noise, maintenance, and convenience are driving it heavily as well. Of all the reasons I love my Model S, fuel savings are pretty far down the list, although I certainly don't mind!
Yeah, that's the thing. If gas ever gets back up to $4/gallon in the US, EVs which are just somewhat competitive at $2/gallon are going to be radically competitive.
However, if EVs become widely used, then the lower demand for gasoline will mean gas prices stay low. So long-term, EVs need to compete with low gas prices anyway, even if prices creep back up in the near-term.
But yeah. Once batteries are cheap enough, then electric vehicles may actually become cheaper than internal combustion engine vehicles /upfront/, in addition to the much lower running costs.
This of course completely ignores the potential effect of self-driving cars. I'm still skeptical we'll see large scale automated driving for at least 10-20 years.