We can't even form a useful moral argument about it until we know its effects
Of course we can. It's ok for people to donate their money and time for this kind of experiment. It's not ok for governments to tax citizens for a new social giveaway unless others social benefits are reduced accordingly (which will never happen).Look at the tax reform debate now: When they proposed taking away deductibility of mortgage interest or state and local taxes deductions, I started seeing headlines from the New York Times and Washington Post about how this a “Middle Class Tax Hike”...
I would compare UBI to the "second system syndrome" with software developers: If only we can start over with an entirely new platform rather than addressing the technical debt in the current system, all would be well.
Every single proposal of UBI has included it replacing all other welfare benefits
Only so that their cost calculations don't go crazy and seem sustainable. No one in the UBI side is even remotely interested in deleting previous welfareI think you're working from some wrong assumption who these UBI proponents are, and what their motivation is. Just to give you an idea, Wikipedia lists Elon Musk, Tim Berners-Lee, Mark Zuckerberg, Sam Altman, Bill Gross, and Tim Draper, among others. These aren't lazy know-nothings trying to get you to finance their lazy Tuesdays.
I don't have any issue with billionaires believing into UBI and sending cash to Kenyans. I don't know their true agenda, but they might gain some political influence / power when they'll try to implement this into western countries (and it will be with our money this time of course, not theirs).
It might not always be smart, or sustainable. Just as it wouldn't be smart not to do it at all. But there's no first principle of morality to reduce to. And if there were, there'd be you'd still need agreement on it for it to be meaningful.
if that's what society decides
So if 50 percent of people are ok with it, then "that's what society decides" so it's ok for everybody ?