Since they have a control group, they can at least learn how people's behavior changes when they get free money for 5 years. 5 years is enough for going back to

Since they have a control group, they can at least learn how people's behavior changes when they get free money for 5 years. 5 years is enough for going back to school, getting retrained, moving to a new location for better work prospects, etc. They can't claim to know how people will behave if it is a lifetime, but evaluating some change is better than evaluating none.