Whatever wrongs you may think of the current capitalist / slavery-like conditions exist for the lower class now, they are not nearly as bad as the hypothetical society that relies on large amounts of labor of low wage immigrants who don't have UBI while everyone else does.

It's arguably the most regressive fiscal system one can imagine. A class of people who must earn an entire cost of living paycheck to net zero with their non-working, unskilled citizen equivalents.