You're referring to 2 different "labor shortages" at once there. The proposed labor shortage in the question "what if UBI were available at a life sustaining level" and the labor shortage caused by the current wealthy being able to opt out of labor. That the latter hasn't caused societal labor shortage issues is true but as you say it has been capturing a portion of the profit in a negatively impacting way and it's also on the order of 100x fewer participants than UBI would have. With that in mind it doesn't seem to be unreasonable to ponder what negative impact expanding such a concept 100 fold may have.