> they can take a hit to their margin instead
Not for long. Low margin businesses tend to go out of business. And if you can earn 5% buying bonds, why run a business that has an ROI of 4%?
Of course, if businesses actually set prices as cost + margin, then it does make sense to separate tax from the price, as the value "price before tax" is a meaningful part of the price setting algorithm.
And lowering your margin is not the same thing as running a low margin business. If your margin was 20% at a tax level of 10%, and taxes go to 15%, then reducing your margin to 18% doesn't make you a low margin business now. Regardless, if you believe in supply and demand as the sole mechanism for price setting, this is all moot: the state increasing taxes won't increase demand or reduce supply with the exact same level as the tax increase, so the price can't move the exact same as the tax increase in this model.
I'd also note that it's not strictly true that taxes are completely decoupled from supply & demand. States do compete on attracting businesses through lower taxation.
That's correct.
> So the tax level is irrelevant to the value of the market price, if you believe in supply & demand as the price setting mechanism.
Taxes raise the price, which reduces demand. If the prices don't rise, then the margins decrease, which reduces the incentive for the business owners to continue.
> doesn't make you a low margin business
It makes you a lower margin business. It reduces the margins on all the businesses, and the marginal (!) ones are no longer viable, and a new set of businesses become at the edge.
When a tax hike happens, businesses don't automatically have to increase prices they charge by the same amount: they can take a hit to their margin instead. So, the price they choose to charge is not "price + tax". It is "supply-and-demand price, of which we pay X to the state in taxes".