A lot of the stuff UBI would pay for is (imo) not that scarce or shouldn't be. Food, water, most medicine, etc. we can absolutely just provide to everyone. We have the resources to give everyone a decent life (ie. not struggling to stay alive, not made artificially awful etc.).
I am quite sure of this for America given the current level of offshore exploitation not changing. I think but am not entirely convinced this is possible globally. Certainly there is some amount of population that is simply too much for the earth to sustainably handle but I have no real idea what exactly that number is.
Stuff that is inherently not scalable then .. yea .. ubi cant fix that. Not everyone can get a mansion, or 1918 wines, or even maybe suburban houses really. Not sure exactly where the lines are but there ARE unique things and it's wrong to discount that.
But the current system is set up so that you have to grind hours doing miserable things largely unrelated to survival to get enough money to _not die_. I feel like UBI is just the simplest most reasonable safety net for that. Certainly there are other options but they add bureaucracy and complications that don't seem worth it to me. It's like everyone has a house they can crash at indefinitely if they need or want to.
But it's not the only option ofc. Just seems to me like a decent combination of simple + politically viable + solves many problems of exploitative work. Healthcare-for-all would be huge step forwards even without UBI so it's not the only goal either.
& as for "right to happiness" - I really believe that most people will still want to be useful and make cool things people like, even if given the option to not. Open Source software etc. is already run this way on weekends. Making something useful for other people will still be an achievement people strive for, and would be rewarded on top of UBI.