The easiest arguments to beat are arguments with the proposition of 'none' or 'all'. Now an intelligent proposition might include the notion that you don't agree with the argument that we should have progressive taxation, but telling us there is NO rational argument is wrong and therefore irrational.
The argument for progressive taxation is that is more likely delivers equal pain to those taxed; an equitable distribution of the burden. Again, our income tax system taxes discretionary income (income after basic living expenses). The progressive system is lighter on those with the least discretionary income and taxes those with greater discretionary income at at a greater rate. The theory is that the "pain" of paying the tax is distributed equally.
Secondly, taxing those at the lower end of income spectrum at a relatively greater rate than those at the higher end is to tax those with a higher propensity to consume greater than those with a lower propensity. The significance of this is that taking money from people that drive demand (higher propensity to consume) tends to slow an economy when compared to taxing those that would otherwise save rather than consume. So, from an economic stimulus stand point, a regressive tax tends to slow the economy.
Now, those are my rational arguments, here are many others:
https://mic.com/articles/3150/three-simple-reasons-why-we-need-progressive-tax-rates#.j2dELzUZV
Economist's View: Why Tax Rates Should be Progressive
Reducing income inequality is in everyone?s interest; the argument for progressive taxes | Northwest Voices | Seattle Times
There is, indeed, many rational arguments in favor of progressive taxation. That fact you do not like the arguments or like the end proposition does not change that fact.
