Why would things be different under the proposed system? We don't have slaves now, and those jobs get done. They typically have higher pay to compensate for the undesirable aspects of the job or risk associated with them. Someone is choosing to do those jobs instead of other more desirable jobs.
In one scenario you have a choice between Y+0 compensation for job J_Y and X+0 compensation for J_X, Lets assume that J_X is the more undesirable job. The proposed scenario is that you would have a choice between Y+B and X+B, where B is a minimum stipend to cover the cost of existence in society. At worst, Y is going to be commensurate with B. The relevant metric is going to be the ratio (X-Y)/B. In the worst case scenario that boils down to X/B - 1. If the difference in pay for the undesirable job was high, then (X-Y)/B is high and you would still choose to do the undesirable job for its higher compensation. If (X-Y)/B was small then the difference between X and Y is small and if you're smart you would be working the more desirable job for slightly less pay.
The only places where B would have a negative impact on jobs is when X/B-1 is negative. Its better compensation for doing nothing. Since B is so small anyway, any job where X/B-1 is negative is essentially exploitation. You shouldn't be working that job anyway as it doesn't sustain your existence.