You definition: “Income inequality is the difference between highest and lowest earners.” The IMF definition as per your source: “Income Inequality, which refers to the extent to which income is evenly distributed within a population”
They’re not the same thing: you picked a very specific metric, not the general definition which is what I was using.
But yeah, if there was UBI that took care of life’s essentials, “one person with a quadrillion dollars” would not matter.
Then again that wouldn’t be possible in the Mercantilistic Economic System we have because in this system monetary tokens (such as dollars) are claims on a fraction of the goods, services and assets of a country, and I really can’t see how it would be possible to both provide for the life’s essentials for everybody AND have somebody with enough monetary tokens to lay claim to most of the wealth produced in the country (unless we’re talking about worthless dollars, such as the ones in Zimbabwe at some point, in which case most people would be quadrillionaires anyway).
Now, if “dollars” were only claims to very specific kinds of things that excluded essentials and things necessary to provide them (which would also mean Land, since that’s necessary for building places for people to live in), then how many such tokens somebody had would be irrelevant, but the closest to such a “money is not needed for essentials” environment we ever had was basically the USSR and we all know just how well that specific experiment worked.
In a centralized state control system “Income inequality” is is indeed not the problem: there the problems are in the production and distribution of goods and services, not affordability (i.e. the scenario where everybody can afford bread but there is no bread), plus centralized state control is by necessity authoritarian, so forget about Democracy in that one, which brings yet another big-box-of-problems.
It’s only in trying to find an ideology to deal with the social side of policy whilst Capitalism deals with trading and resource provision, that you end up with the wealth concentration problem: if dollars are claims to goods, services and assets, then the fewer the hands holding those claims the more the State has to tax them to provide the essentials to everybody and as you might have noticed, those with lots of money use it to buy legislators and agents of the State to make sure they’re taxed as little as possible.