"Idle hands are the devil's workshop." The more people that are economically sidelined, who nave nothing to do and no prospects for self-advancement, the more appealing radical ideologies will become and the more social unrest there will be. A major source of social cohesion is the belief that if you work hard you will be able to succeed, but if it's structurally impossible to find work, and all you have is a UBI which doesn't afford much, you can expect that mass stagnation to provide ample breeding ground for a colorful assortment of social ills--drug abuse, suicide, crime, and finally, once the individual angst finds collective expression and organization, revolts and political radicalization.

Work is one of the major sources of purpose people have. Earning an income and putting food on the table is fundamental to one's self-respect. Like educational achievement, it gives the impression that there is some way to advance beyond your lot. Take that away and it will cause a mass existential crisis unless there is something to fill the void. All the blights of the rust belt and other economically overlooked areas should be expected to spread.