Denmark already have a limited form of the UBI when seen in that light. The state offers free education and a monthly "basic income" for people studying for a degree, for up to five years.
Sure, some people waste it. Some are just passive consumers, even shopping around between multiple educations without ever completing a degree. Some drop out half-way. Some get impractical degrees with few real job opportunities.
But enough people go on to become doctors and engineers and software developers and so on, and then have long careers that ultimately pays back the venture capital to the state, in the form of taxes. Most also work a side-job while studying to supplement the "basic income" stipend.
I don't personally believe that the majority of people will become unproductive consumers with an UBI. I think that societal pressure to contribute, the wish to enjoy luxuries, and to get status is enough for the majority to still work. I think that the added safety net of the UBI will also allow more people to take a risk on a dream, and perhaps make it big in art, in inventing new stuff, in science or in politics. And, as in VC investments, the few big hits will hopefully pay for the failures.