The goal of society should be to become post-scarcity. All of our technological progress has led in that direction. We currently just live in a period where the majority of the gains have been captured by very few people and that has allowed them to exploit human labor to enrich themselves vs. building a self-sustainable society. Engineered or not, I think that's the inevitable end game of continuing humanity.
Apart from needing those technologies to exist before being able to claim them as reasons there won't be a labor shortage with UBI this is still completely independent of rich people existing today without causing a labor shortage. Perhaps it's a funding source to prove out things like automation by redistributing said wealth but the existence of said wealth in the hands of a few is not prove lack of labor in all is sustainable. To do that you must actually prove the alternative means of production out - regardless if the wealthy exist.
The problem is that we are dependent on these zero sum games to regulate our economy and as a result these zero sum games end up consuming all our our productivity gains and making up a progressively larger share of the economy.
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.