The supply and demand of labor does not care what people need - it does not care at all that mothers want childcare, or the citizens want better roads. All that matters is if that mother or if the citizenry are willing to pay for it.

Predominantly most of that is due to the labor glut driving down median incomes and leaving people with less economic negotiating power to get the things they want done.

It isn't appropriate to talk in absolute figures of reducing labor supply. That is one side of an equation without considering the other. UBI would generate tremendous real demand for cheap housing, cheap healthy food, and all manner of amenities that can fit in that universal budget. Those demands would be met, because everyone than has the economic negotiating power equal to their UBI as a minimum, rather than today where many people have no (unemployed) or negative (indebted) negotiating power at all and thus have no effective economic influence at all.

It is always about supply and demand, and to only talk about economics in terms of one or the other is to argue for an intended outcome rather than seeking out the best possible one.

If there were actual demand for childcare, the going rate of childcare workers would continuously rise as capital fought to be supplied by limited demand. The rising prices would create incentive for people to enter that labor force. That does not happen because real economic demand does not exist at those levels - it is not an actual labor shortage if all you want is childcare workers at minimum wage. If I want rocket scientists at minimum wage, I cannot claim there is a shortage of rocket scientists period if they are not willing to take the lowest possible payment I can give them.

There is a shortage of something when the real supply cannot meet demand at sustainable price points. Are Nannies expensive? Yes. Is that not a sustainable cost? Absolutely not, Nannies are a valuable commodity, and those that can do a good job should be paid for the scarcity of their skillset. The reason there are not more Nannies is not because of some incredibly bullshit argument about how "nobody wants to do the job, we would pay them anything, but people just refuse to act rationally economically" it is because nobody is willing to pay anyone sufficient wages to justify the career pivot into the discipline.

And that relates back to my central argument for UBI - our economy prioritizes the work "worth doing" as a central tenant of capitalism. All a UBI does is potentially drop out the labor force of the work being done now that is the least worth doing to begin with.

But TLDR, fundamentally, it is important to recognize that in capitalist economies, demand is not people wanting something (better roads), it is the money willing to be spent on the supply. So if nobody will supply the money to build better roads at rates construction workers are willing to take, you do not have a shortage of construction workers, you have a shortage of funds to pay for better roads and thus you are not generating enough demand for them.

We have a demand problem, not a supply problem, as long as major corporations are hoarding money and the stock market is at all time highs, capital is out there. There is just nothing the controllers of that capital actually want that anyone who isn't working now could supply.