Why would each taxpayer have to purchase the same set of goods? That's nonsensical as it has no bearing on how the world actually behaves.
In life as someone's income increases their consumption increases. This can easily be seen anywhere in the world. Therefore as someone's income and consumption increase they will buy more things, increasing the share of taxes paid as a percentage of their income.
Your example only makes sense if we live in a state of antlike conformity where people's consumption preferences are static. If that were the case what would be the point of making more money anyway?
They dont have to actually purchase the same set of goods. However, you have to assume they do to calculate whether the tax is regressive. Otherwiseyou, are comparing apples and oranges by changing the constant when attempting to calculate the variable.
