As you said, it’s inherently arbitrary where someone draws the line at what they call rich, middle, or poor.
And certainly in fact an idea of sharp cut-offs just doesn’t make sense. Obviously, it’s a continuum.
Personally, and I suppose perhaps many see it the same though I don’t know if they do, I suppose most would consider a person rich if their assets are so large that their investment income alone allows them to live a lifestyle which is beyond that affordable to the merely professional class such as say most salaried-or-working-under-contract doctors.
Middle would be being able to afford your own home, support a family in reasonable comfort, own a reasonable vehicle, and own and enjoy common goods and service.
Poor would be be having a hard time paying the necessities.
Those in the gaps would be somewhere mid-category.
There is no reason that people earning enough to be able to pay for their necessities should not pay at least some small share of income beyond that point, assuming others are being charged income tax.
There is no reason that those earning higher incomes should have to pay not only the inherently higher amounts from paying the same percentage on larger amounts, but on top of that should have to pay a many-time-higher percentage. To do that is to demand they carry an inequitable percentage of the total burden.
Probably the most equitable thing, if having an income tax at all, is a flat rate on everything past some amount required for necessities, and allowing deductions for necessities such as unusually high medical expenses. It also would be equitable to allow other deductions for purposes of promoting desired change, for example, allowing deduction from income of health insurance costs to promote people buying their own health insurance rather than going uninsured, etc.
So not necessarily an absolutely flat tax, but not a bizarre system such as we have today where, for example, in the 2004 election it was revealed that the Kerry’s paid only 16% on a claimed income of $5 million, with that percentage not only being far less than for example what most of us pay, but obviously, given their lifestyle and the fact their assets are over one billion dollars, the $5 million figure was a completely on-paper-only feat of accounting in the first place. Nope, not that sort of deduction schedule.