Landlords and groceries can just raise prices to bring the cost of living up
They already can, and do. If they do it too much, people leave that area. With a UBI, there’s nothing that says you have to live in a big city, it would be easier to move to bfe, where it’s always going to be cheaper. It’s not ideal of course to uproot and leave, but it’s possible, and it’s that possibility that keeps prices somewhat under control.