I've been thinking about this, and I don't know if a UBI like people envision will ever really be viable.
BUT as society gets more and more wealthy, I'll bet that living on the fringes of it will be more and more viable
We're already at a point where you can work minimum wage for an hour (take home ~$7) and buy a few days' worth of subsistence-quality food if you have access to a kitchen. You can pare down your other living expenses too, and suddenly you have a reasonably comfortable lifestyle that only costs a few hundred bucks a month.
Right now we stigmatize "van dwellers" and couch surfers, but maybe that'll change in the future