Funny thing happens when labor supply drops...prices for labor go up. In this case, the market would determine how much they'd need to go up by to get enough people doing things to meet demand. It may be more than many want to pay, but such is life.
Look at the response to a higher minimum wage - McDonalds, which typically runs a store with 8 to 10 people at $7/hour discovered that most of those jobs could be automated (with better results) for the capital expenditure equivalent of less than $15/hour. So now there are McDonalds stores that run with 2 people instead of 10. People still get their hamburgers, but now orders are taken by a computer screen, hamburgers are assembled by a machine instead of a person, etc.
Lawnmowers, street sweepers, taxi cabs, factory lines, farms can all be nearly fully automated these days, but it's cheaper to put a person there right now.
This will produce a constant back-and-forth impact on labor markets as jobs are automated, then new jobs are created as a result, and then those jobs are automated. Having the basic income in place allows society to remain functioning while this happens and makes it more able to weather the changes and adapt.
FWIW I'm in favor of a UBI to replace almost all other assistance programs.