A UBI would certainly be more beneficial than "prosperity through employment" measures during a pandemic that has shut the entire country down. It's kinda hard to have economic prosperity when everyone is hunkering down at home. But why can't we have both in the long run?
I think we have two monumental issues that we are dealing with at the same time here.
One is "the machines are coming" whether we want to admit it or not. Actually, "the machines are already here" is probably a better way to look at it. Machines can do the work of 1,000 people in a fraction of the time. Machines don't take lunch breaks, weekends off, or vacation time. They don't get hurt on the job, can't sue for wrongful termination, or quit and move on to bigger and better things. Manufacturing is thriving in the United States. It's just being done by robots and not humans.
The other is this pandemic has taught us that people already have jobs. Unfortunately those jobs are in the countless small businesses selling niche items, the restaurant and greater service industry selling us our favorite food and booze, and other "service" jobs that cater to peoples everyday whims. They are already employed and getting by in life in the best way they can.
A UBI would certainly help those people a hell of a lot right now. As would a living wage, but that's an entirely different subject for when the pandemic is over and people can actually work again.