Somebody suggesting nobody would want to do construction is probably more indicative of a person who has never had the privilege of ever building anything in their life, rather than any statement of reality. However, absolutely awful examples aside - I do think he is correct that there are plenty of jobs that indeed nobody would want to do. Toilets don't clean themselves, and won't reliably be doing so anytime in the foreseeable future. There's no fulfillment mopping up piss from somebody who thought it'd be fun to urinate all over something, just because it's not theirs.

Of course you're right that we'd see pay go up for these jobs as a result, but I think the thing that many don't consider is how relatively little money there is to go around. This is masked by scale. We see billions and think of just enormous amounts of money, yet that's of course less than $3 per American. That of course means the even more unimaginable trillion dollars is merely $3000 per American. We have so many people that it can be difficult to really intuitively grasp.

So one of the most important numbers here is the GDP per capita. The GDP being the total market value of absolutely everything produced within a nation over a set period of time. This is what makes GDP per capita so interesting. It tells us how much money each and every person would receive if the market value of absolutely every single good or service was split completely equally. And in the US it's shockingly less than $60,000.

With an absolutely huge motivation to overproduce everything imaginable, and a countless array of artificial demand being created and sated, our GDP is still less than $60k. That's not a particularly huge amount of money! And while we could debate to what degree, I think nobody would disagree that an UBI would depress overall production. So we're not even going to be getting to that $60k point, and probably not even particularly close to it.

And this amount of money needs to be used to provide a livable stipend to each and every person, and then also account for 100% of the market driven economic success for each and every individual. I'm sure you can see the problem. Exactly how much money is our janitor supposed to be earning? And where does this money come from? You have to keep in mind that when we talk about this < $60k GDP/capita this isn't "money" but rather real money that represents a share of access to a finite set of resources. In other words you can't just go above this number and call it debt - the resources that such "money" (as it would become at that point) would represent literally do not exist. It'd be like in a world of 100 with a total of 5 cars telling each and every person that they can have a car. It just doesn't work.