How should authors get paid? What about editors?
Look, no artistic endeavor needs to make a billion dollars. Even when something does take a billion dollars in revenue, none of it goes to the artists anyway! Music labels and movie studios and book publishers are infamous for creative accounting and bogus contracts and outright lies to fuck over almost every person that actually contributed to the art. They're worse than politicians. It's all there to funnel even more money to people who are already rich. If you put artists on basic income you're not paying them any less, you're not losing anything, 99.9% of them come out ahead of where they would've been if they'd sold their work for money. Maybe add a couple more nines to that list. For every multimillionaire movie star or generation-defining author there are literally hundreds of thousands of artists just barely scraping by. Capitalist art only benefits the leeches.
If it is not enough to live on, then only the independently wealthy can do art, which goes against the exact goal you stated.
Also the idea that a significant UBI would lead to people just coasting seems patently false. Every bit of evidence shows that when given the opportunity the majority of individuals are driven to improve the quality of their lives; and also, that quality is measured in relative and not absolute terms.
Or alternately, millions of people will cease busy work which adds no value - and engage in genuinely productive tasks instead. Caring for their families, locally growing food, building collaboratively, teaching and learning form one another. We only reach for succour and addiction when dispirited and alienated. Graeber was far too conservative in his definition of bullshit jobs. The vast mass of us now work in the production of bureaucratic services so far removed from an actual good as to be incalculable. Our work is bullshit, or worse - actively destructive, and we know it.
Most people want more than this and will work for it. But accepting it as a form of sacrifice to achieve your other goals is hardly a problem -- and then people can create their art or start their own business with sweat equity before it generates any revenue etc.
Artists would then get the UBI and whatever they get in terms of patronage, donations, art sales, etc. Neither of which might be enough to live a middle class life on its own, but both together makes it happen. And then the people who make a go of it but never manage to produce "art" anybody cares enough to support at all never make it out of the tenement and retain that incentive to do better or find a different path in life.
And importantly, they continue to have their basics covered while they do that. Part of the reason most people end up stuck in bullshit jobs is trying for something better is too risky.