Discussion of a robot tax should consider what alternative we have to deal with rising inequality. It would be natural to consider a more progressive income tax and a “basic income”. But these measures do not have widespread popular support. If support is not widespread, the tax, even if imposed, will not last.
When taxes on high incomes are raised, usually in wartime, it turns out to be only temporary. Ultimately, it seems natural to most people that taxing successful people to benefit unsuccessful people is demeaning to the latter, and even the recipients of the handout often do not really want it. Politicians know that: they usually do not campaign on proposals to confiscate high incomes and pad low incomes.
So, taxes must be reframed to remedy income inequality induced by robotisation. It may be more politically acceptable, and thus sustainable, to tax the robots rather than just the high-income people. And while this would not tax individual human success, as income taxes do, it might in fact imply somewhat higher taxes on higher incomes, if high incomes are earned in activities that involve replacing humans with robots.