I'd word it slightly differently. I agree there may shrinking pool of economically viable jobs and it's likely to shrink considerably in coming decades.

However, there's lots and lots of stuff that needs to be done for society to function properly. E.g. all sorts of community work. From organising local community events to self-support groups to picking up trash in a local park. A lot of stuff works (or doesn't, looking at mental health statistics) just because a volunteer pops up. However, if we as society suddenly had a fuckton of money available, maybe we could improve here? Hire people who can run arts classes in local community center. Or, first, build the damn community center :) Or hire more people to pick up trash.

It's not economically viable to keep streets and parks clean. Or offer free arts classes. But if we don't care about what is economically viable, maybe we could put resources there?

Of course, with UBI, maybe someone would volunteer to pick up trashes while getting a meager amount of money. Or maybe he will sit on a couch because picking trash is a boring compared to playing Guitar Hero 24/7.

However, building strong communities with passionate people may create all sorts of issues in the future. It's easier to control couch potatoes relying on government handouts for generations.