[pm90]

They already do, to a large extent. I guess the OP should define what a comfortable middle-class lifestyle really is. If I had to hazard a guess, I would say: capacity to buy a decent house and car, pay off present and future debt and stay out of it, enough disposable income to meet life's necessities and yet have enough left over to save for a comparable lifestyle during retirement.

I don't think the UBI would provide all of that; instead it would be a means to avoid destitution (e.g. starvation, homelessness, unmet medical needs etc.) in the present. For all the luxuries that most of the middle-class hopes to achieve (luxury car, nice house, investments, vacations abroad etc) you would need more money than UBI.