That’s true but on the other hand if you never spend money then it’s not providing you any advantage. Money is only useful if it’s spent. In that sense a flat sales tax is the “fairest” since it reflects the actual chosen consumption of that individual.
(I know this isn’t true in practice because of exclusions from sales taxes, but we could imagine one that applied to all consumption.)
No, another thing money does besides fueling consumption is fueling investment, which generates yet more money.
There is some strange axiomatic derivation happening in this thread. The term "regressive tax" has a specific meaning; we can just start with that as an axiom, rather than re-deriving all of economics. A regressive tax is simply one that takes a higher percentage of income from low-income taxpayers than high-income taxpayers.
(I know this isn’t true in practice because of exclusions from sales taxes, but we could imagine one that applied to all consumption.)