Poor people are poor. They don't have money to afford things. That's what it means to be poor, and that's who we're talking about when we're talking about poor people.

So it is a matter of definition that poor people cannot afford to decrease their carbon footprint. I've said nothing to compare poor people to affluent, or made any argument dependent upon demographics whatsoever. (In fact, I've been arging that the only thing safe to assume about poor people is that they are poor.) And poor people cannot afford new cars and homes.

Given this, they will not be getting new cars and homes unless someone else pays for them. If you give them a tax credit and then immediately charge them for having inefficient homes/cars, then you haven't changed anything. They still can't afford new cars or homes, because they are still poor. Certainly they will not spend their <$5000 on highly efficient cars and homes.. both because this is not enough money for such things, and because they have more pressing needs to spend that money on.

The only way a tax policy will not be regressive is if you give more money to poor people than to rich people. Giving people discounts for taking specific actions does not achieve this in any way, because rich people always have more options, more easily taken, than poor people.