I don’t think “This other, largely unrelated, problem is bad so we shouldn’t do this thing” is good reasoning.

I don’t think in the real world, in all places (or even most places) all the stores are in a cartel. Where I live, there are several large supermarkets and a handful of smaller groceries all within walking distance. They are not a cartel. They compete. You’re just making stuff up for some weird dark fantasy of yours.

Furthermore, if there was a monopoly, and we have the political might to implement UBI, I dare say we’d also have the political power to do a tried-and-true popular move of breaking up monopolies.