Why is that the best case? In my “best case”, all of everyone’s needs are met, people pick and grow more intense crops because they want to, artists just enjoy making art, and “income” from work just isn’t really a thing anymore. My best case is everyone has what they need and can access what they want.
keeping these things going when people lose interest will be a challenge.
In a heavily AI-oriented future, why wouldn't that be done by the floor-cleaning robot?
Though I suspect on a long enough timeline, that job will be automated too and UBI will become a "here's a [weekly|monthly] stipend to live life on, but it's not too much to act truly foolish with."