Because poor people can't spend money (since they don't have it), hence they pay much less (or even no tax). The curve of a power law formula can be modified to whatever is socially acceptable. Otherwise, rich people who are hoarding assets will get taxed via land value taxes utilizing the same framework.

>you've still got problems of hiding how much I'm spending through various legal entities.

Same with income tax. Completely stamping out tax evasion is not a realistic goal with any system. Also, various legal entities are all tied to beneficial owners with taxpayer ID numbers. Databases would make quick work of sorting this kind of stuff out.

>It also has a macro economic problem, at least how the consumer economy is structured, that could slow down the gears of business until society realigns (if it does) because people will be incentivized to consume less.

Yes, that is why it is a pipe dream. But it would actually accomplish environmental goals as opposed to just pay lip service and pretend.

>You've got alternative models where you go after specific products and tax more for conspicuous consumption like yachts but that feels like it has all the inefficiencies of central planning a market.

Completely unnecessary to complicate things which also opens up avenues for corruption. A yacht is super expensive, it's obviously going to be hit with a ton of tax. Just come up with a tax curve (like we already do with the various income tax deductions/brackets) that provides the tax revenue and still allows people to spend enough money to provide for their basic needs.

I'd go with same tax rates for everybody, but tax credits for useful activities. Since being a poor consumer is useful activity because those pay most attention to what they buy I'd be just giving people tax credit for existing and possibly for working if I wanted to incentivise that. Since purchase tax for individual consumers should be collected and paid by the sellers to save the individuals the burden of it and to ensure compliance then the best form of tax credit for consumers is just cashed given them directly from the government. Some for existing some possibly to supplement their salaries if they work. There's some risk that they won't spend it all to pay for their purchase tax but this can be forgiven as it'll mostly happen for the very poorest.