Yeah, that's kind of the argument, but it's not a very good one (I know it's not yours, bulletproof). Have you seen average income levels of LEAF buyers? There's sure a group that doesn't need an incentive to be able to afford that car.
However, the point is not to make the cars affordable - which is why the argument is a poor one. The point is to drive buying behavior, which we know incentives do.
Incentives and buying behavior
Some people will buy with or without the incentive; others won't - just like any other incentive, including toothpaste coupons (too bad it's impossible to only offer incentives to people that wouldn't buy without it). The question is, how many. That's an extremely difficult question to answer; one thing I'm sure of is that income doesn't have a whole lot to do with it, at least not above a basic affordability level (which is why proposals to tie the incentive to income won't help either, as the incentive is not big enough to materially change the number of people that can afford the car. Plus it is bad policy to encourage people to buy something they can't quite afford. And it makes the process more expensive to manage, and more intrusive and confusing which will reduce purchases as well). Toothpaste manufacturers have data showing their coupons drive sales and increase profits despite lower selling costs and everybody being able to afford their product already. (However, they very carefully calculate the optimal incentive amount).
Whether you have an incentive or not, most buyers will be well-to-do, the key question is how many of them will buy it, not who can afford it. Incentives very clearly drive buying behavior - more cars are sold where there are incentives, no question. And it pays off - CA can spend money in other ways to solve these problems, but it turns out that EVs are actually a fair bargain given how many issues (particulates, groundwater runoff, CO2, out-of-state petroleum dollars) they address.
Capped incentives
I do support capped incentives, but not an all-or-nothing cutoff, and especially not one that rules out products that help meet the goal. For example, even though the Model S can be considered a luxury good only bought by the rich (but why is that bad - don't you WANT the rich to help fund this transformation? Who else will buy expensive new technology - there are risks, after all? You don't want to subsidize this forever; what's the fastest way to make the technology cheaper?) it does have a relevant non-luxury capability no other car has - 265 miles of EV range. Not to mention AWD and 7-passenger seating. Some people are buying a Model S that simply couldn't drive so many electric miles in any other vehicle.
Once 250-mile $30k EVs are available, my opinion will change on this subject. But for now, it's still important to sell cars like the Model S. At least the lower-end models of it - I see little reason to incent P85D sales. (There is still the argument that P85D buyers may be replacing gas guzzlers and wouldn't settle for an 85D; but there's also an argument that those buyers are least likely to base decisions on incentives).
Social justice
A related often-used argument is that poor people should be able to enjoy EVs too. While I'm all for more social equality (I believe that's good for rich and poor alike), I don't see how these incentives tie in to that. Sure EVs have nice driving characteristics and can save on fuel costs, but poor people can't make the up-front cost tradeoff - heck, they don't buy new cars at all - and the incentives are not meant to address social inequity. They are meant to improve the economy and (especially in CA) reduce pollution; those effects happen no matter who buys the car.
Summary
Taking incentives away from the rich sounds like a good thing. And I readily acknowledge that some buyers (it is hard to say how many) would buy without the incentive; it could be that the number turns out to be large enough that taking away the incentive is indeed a good idea - I am willing to investigate this sort of propsal. But we don't have all the data yet; and rather than trying to get that information, we are seeing feel-good arguments being used to push this change that miss the whole point of having the incentives.