My genuine experience is indeed that “ nearly all of the most vehement opposition to Basic Income comes from people who are themselves relatively wealthy.” I don’t claim or assume that is everyone’s experience.
As to your claim that “Basic Income would deprive productive individuals of the right to the rewards they produce — and grant to the incompetent, the stupid, the slothful a right to the effortless enjoyment of the rewards they could not produce, could not imagine, and would not know what to do with or how to live without,” — that itself sounds suspiciously like part of a strawman argument.
After all, people who accumulate money and have to pay taxes are not all very productive individuals, do not all make a very positive contribute to society and may not entirely deserve the incomes they receive or the wealth they accumulate.
And the people who would rely on Basic Income are certainly not all “incompetent,” “stupid” and “slothful.”
If either one us is guilty of relying on strawman arguments, I politely suggest that it’s probably you!
And when you say that “The moral argument against Basic Income is rooted in the observation that it is unjust,” I would say that ‘observation’ is more like your personal, highly-subjective opinion and not really an ‘observation’ at all.
And perhaps you would like to add a useful data point to this debate, by answering this question: Are YOU a relatively wealthy individual?