Sure, so ... who do we force to do the janitorial work once nobody has to work anymore? Or does the need for it magically vanish? If the answer is "we pay them more", great. We're paying more to janitors, janitorial work gets more expensive, we need to make more to afford it, we need more UBI, janitorial work becomes unattractive.
You'll have to give up a significant part of your (real, not nominal) income to make a difference there. You might be happy to do that, my guess is that most of your peers are not.
If I give you 1k, you get some amount of spending power x, if I give everyone 1k and you're on the lower end of the income scale then you've got an additional spending power y. 0 < y < x - getting UBI offers an absolute and definite increase in spending power to those in need, it doesn't give them that spending power at 100% efficiency though, it just shifts some spending power from the richest to the poorest.
The reimbursement of janitors might go up with UBI, that's fine, if it's going up then it means that something in the current market situation was artificially holding back their reimbursement. The market can correct pretty easily for UBI and make sure that the jobs that need to be done are indeed done (or else they aren't jobs that need to be done) and maybe janitor, as a profession, is scaled back to just waxing the floors and we discover that people working in an office should, once a week, empty their own trash cans - it's a very plausible outcome from UBI and one that will happen naturally if it'll happen at all.
If you let the market respond, it will just price in the UBI, and you'll end up with pretty much the situation we have today, only with prices increased by some amount and some added bureaucracy.
> maybe [...] we discover that people working in an office should, once a week, empty their own trash cans
They can do that today, it's just more efficient to pay somebody else to do it than to have you, a highly-paid software developer, do it. It's more efficient to have you do complicated software things than mopping floors and emptying the trash can.
I'm all with you that there's a disconnect in today's wages where high-skill knowledge workers get out-of-proportion amounts of money while low-skill manual labor is basically priced at the minimum amount you need to survive.
I don't believe that UBI will solve that if you keep everything else (that is: markets) the same. And I don't see any compelling argument that some (new or old) alternative system that you or I or some people that are way smarter than us could come up with would work more efficiently than our largely-self-optimizing market system. So I'm not a fan of "well, out with the markets if they are what's holding us back", because the risk of it failing catastrophically are just too high to go "well, maybe it'll work this time".
I'm happy to be wrong, and I'm far from an expert on any of this, but there's too much hand-waving in UBI suggestions for my taste. Some proponents seriously claim that "doing away with be bureaucratic overhead" would be enough to finance UBI, and I'm dumbfounded that they haven't even done any rough guesstimates of the numbers to realize that they are wrong by orders of magnitude.
Controlling inflation will be a massive issue, and I don't haven't heard any convincing answers.
If you find a way to convince the super majority that "more more more for me me me" isn't going to make them happy in the long run, that more cooperation etc will benefit everyone (including themselves), the chances are certainly better, but let's find a way to convince them first and then change the system and not change the system and pray to any and all gods that they will become convinced to avert a catastrophic failure.
You'll have to give up a significant part of your (real, not nominal) income to make a difference there. You might be happy to do that, my guess is that most of your peers are not.