B is for Basic, not funding your passion projects. Enough for simple food and housing somewhere relatively cheap. You want more? You work. Work not working out? You can fall back to the basics without being afraid for your survival.
If it's enough to enable you to do what you want without working, then it's enough. If it's not you find a way to get more.
There are a lot of problems with people in tough situations where their outcomes are made worse by having to make bad choices. People accept abusive living or working situations, suffer through anxieties and trauma because it's a choice between an awful situation and going hungry and homeless. A whole lot of people are wage slaves. The "option" of working, where to work, and to accept situations is only an illusion because the alternative is much worse.
Give people UBI instead of difficult to navigate social programs which don't guarantee support and you have to be an expert to navigate well and you solve a lot of problems.
People living on the street are expensive to society and not in some abstract way. People have less lifetime value to society because they are forced to accept their current situation and don't have the resources to have job mobility or can't afford to put themselves in a better living situation. People also treat jobs like they are forced to do them. A workforce that is there by choice performs a lot better, gets treated a lot better, and has better outcomes for customers, employees, and in general everyone.
Social security wasn't meant to fund retirement, it's a safety net to prevent the elderly who can no longer reasonably work from pushing way past their limits or starving. It wasn't meant to fund your expensive suburb lifestyle. It isn't luxury, but indeed you can live off of social security but it requires choices that lots of people don't want to make.