The air quality excuse is _VERY_ dependent on where you live. On average (
https://www.eia.gov/tools/faqs/faq.php?id=427&t=3) ~30% of the power in the US is derived from coal, another ~30% is natural gas. So basically for cars your just relocating where the exhaust happens. And in the case of coal the byproducts are massively more harmful and long lasting compared with the byproducts from gasoline (which is really quite clean at this point). So you then have to consider the efficiency of the generation/transmission/charging/driving which isn't so much better that electric cars are a mirracle cure for global warming/etc. Particularly in the case of tesla, which has been repeatedly shown that in most places in the US actually polite more than a modern econobox mostly due to being such a heavy car.
So, beyond that, I'm sort of irritated by tax subsidies, because they are really no different than paying someone cash because someone else has to makeup the shortfall, be it through worse education systems, or simply paying more taxes. (I'm also in the camp that believes toll roads are just a convenient way to raise taxes while simultaneously claiming to cut them).
But back to electricity generation, electric cars are _NOT_ an insignificant load, and further subsidizing them to the tun of tens of thousands of dollars in electric charges (assuming a ten year lifespan for the car) on the backs of the common ratepayers, is about as regressive a tax as is possible. The same is true of the loss in gas taxes. People who can afford $100k teslas (teslas are well over 50% of the electric car market in the US given rough estimates, the leaf being the other significant player with ~30k cars a year) don't need further subsidies on the backs of people who on average are earning less than $50k a year.
So, yes the discounted rates are the final tipping point, $7.5k a year in tax incentives + 1-3x that much again over the life of the car paid for generally by those that can least afford is is the worse kind of policy.