We used to have this Enlightenment ideal of a Liberal Arts Education. It served as job training, sure, but it also tried to make its recipients better citizens and more adept thinkers and more capable adults and, broadly speaking, more effective human beings in a bunch of socially and culturally important ways. I'm optimistic that free college tuition would be a net gain for the economy, but I also strongly suspect that it's worth doing even if it costs us some economic growth.
Besides, if someone wants to take a 4 year vacation, they're already well within their rights to just not get a job and do whatever they like with their early 20s right now, presuming they can cover their living expenses somehow. Free college tuition doesn't actually allow anyone who wouldn't otherwise be able to to slack off, but it does give them something socially positive to do with their slacking.