people on benefits always find constructive things to do with their time, they never get depressed due to lack of purpose and end up on drinks, drugs or in prison.
You're not thinking it through -- the goal isn't just to put everyone on benefits and make them spend the rest of their lives clicking the TV remote and waiting for their next welfare check. If you want to do it right (and the robots provide sufficient surplus resources to support it), you go a step further and hire people to do the job they always wanted to do, whether it makes a profit for anyone or not. If that means we have 100,000 ski instructors and 300,000 mediocre artists, then so be it; the robots do the grunt work, and the people are paid to do their preferred avocation.
Not that I expect that to actually happen, of course; in the event the robots actually can replace all labor, the upper classes will make sure that economic surplus goes to themselves, with only the absolute minimum getting distributed to anyone else.