Increasing tax rates beyond a certain point (or retaining tax rates at that point) is counter-productive for raising further tax revenue
Agreed.
The disposable income of various taxpayers after they've paid their taxes is simply not relevant in this regard.
Not agreed, and your next point illustrates that disposable income is relevant.
Say incomes above a certain level attracted tax at a rate of 100%. Nobody would have any incentive to earn an incomes above that level so the economic effect of a 100% tax rate is to decrease the tax take at that level to zero. As such, tax revenue is maximised a rate that is somewhere between 0% and 100%.
And the incentive is diminished on the basis that disposable income is excessively reduced. Disposable income is relevant.
But you seem to be arguing something different than what the Indo article outlines.
I have no issue with agreeing that paying 40% tax on income after €33,800 is excessive.
I have no problem agreeing that USC, while the principle is broadly progressive, the rates applicable are somewhat regressive.
What I take issue with is that the article suggests low income earners are getting a free ride.
Here is a suggestion, if lower earnings provide for a free ride, take a pay cut!
That of course wont happen nor is it desirable or feasible and I do not advocate it, but merely to illustrate the defunct nature and the futility of the Indo article.
The apparent solution proposed by the author is to transfer tax liability from higher earners to lower earners without any additional supports or without factoring in the cost of childcare, rent, electricity, groceries, motor tax, car insurance, mobile and internet technology, TV license etc, etc.
The cost of these items will take a disproportionately higher % from a low paid workers income than a higher earners income.
That is not to recognize that workers are being squeezed. As someone in a two income household with two kids I am fully cognizant of the limits of those pay cheques to meet the bills. But I fail to see how if I reduced my income (and thereby my tax liability) how my lifestyle would be any better. Put simply, it would not.