Let's be frank.
Proponents of a UBI simply want more welfare and redistribution of wealth/income.
Yes. That's what all other stuff mentioned here is about, too. Practically every political decision is about the distribution of wealth.
They say a UBI will be cost neutral -
No.
The distribution is a money saver compared to current systems, but not the UBI itself - as you have said yourself in the first quote.
btw. regarding the inflation topic:
If some do not do the cleaning work now, and instead the people causing that work clean themselves, it would be a deflation. You look way too one sided at it.
Would a certain part of jobs become more expensive? Certainly yes. Putting aside if that is offset by the UBI, that is not a bad thing in itself. It is simply a different redistribution of wealth - from capital income to work income, from the tendency.
Also some jobs could or are even likely to become cheaper.
But in the end, how it will work out nobody can say for sure before, as it was when slavery ended, which would make it impossible to $insert what you want