(1) people work out of pride, not necessity. if your only concern was sustenance you could live off of welfare or charity now. even with a basic income, people will still be motivated to work because they want to have nice things
(2) market forces will still apply. if you raise your prices because low income customers now have more money, I'll lower mine. barring cases of outright collusion, you'll see economic growth as opposed to rising prices
Basic Income is still a long way away, but it has been thought through. A practical implementation may or may not succeed, but it won't be because of fundamental flaws in the idea.
(1) The way jobs are valued would fundamentally change. Rather than higher wage work being the work that fewer people can do, you would also see a rise in cost for jobs that people do not want to do. Today you have your unskilled employees trapped between a rock and a hard place when you offer them a job as a burger flipper - work in wage servitude for us, or starve. The employer has a tremendous amount of sway during negotiations in an economy where employment is effectively mandated to survive.
Ask yourself - "why is McDonalds so cheap?" does it make any sense that having people working in depressing service jobs makes cheap food? Wouldn't the optimization of food be automated delivery, minimizing the people involved? McDonalds can only exist as it does today because all its employees are forced to work for it.
(2) You would also see production shift. Economics goes where the money is - we have too little low income housing and too many McMansions because single family houses are profitable and reliable when sold and low income housing is unreliable and less profitable. If everyone had a minimum of income, there would be a tremendous amount of economic pressure to meet the demands of that crowd that has maximal economic consistency in revenue. So in the short term prices would be a mess, but in the long term the equilibrium state would be that there would be a huge market to provide as much of what the UBI unemployed class wants as possible within their budget for profit.
2) Changing the minimum wage also adjusts the prices of goods and services but in the end it still ends up being a net positive for society. When the minimum wage is increased prices may go up at the same time but usually they don't completely cancel out the wage increase.
If it's the case that there simply aren't going to be enough jobs for everybody then this would be an intended outcome.
What won't last very much longer is this antiquated notion that two thirds of the adult population "should" be working. It should probably be more like 25% now, and that will only decrease. (NB: it could be 50% working half time, 75% working one-third time, etc.)
This notion of people having "jobs", and that being a normal thing, is fairly new in the story of H. sapiens (and it's only reaching parts of the developing world now, right as it's becoming obsolete).