UBI would allow people to expand their career paths since basic survival would not be on the line. Personally I've got minor health issues from being in front of a computer too much, would absolutely love switching it up with a more physicaly demanding job if I had UBI to make sure a drop in wages wasn't threatening my family's basic survival. This also assumes universal healthcare.
"You cannot have both an opt-in working class and an opt-in nonworking class and have a healthy society."
So I agree, but I do not think it would become a real problem. If it does become a problem we could implement a lottery system that swaps workers out every so often.