I also note that you conveniently "forgot" to address those who legally earn their entire incomes from tax free sources.OK, do you want to talk income, AGI, or taxable income? My opinion is that income/payroll is the base of the income/payroll taxes.
And my opinion is that the source(s) of one's income should not be relevant to one's duty to pay his fair share of the costs of government since those who earn ILLEGAL incomes interact with those of us who earn legal incomes in every manner imaginable.