I don't personally advocate for UBI, but I'll counter your question with another question; how are individuals supposed to have class mobility in an economy where the majority of transactions are speculative? The traditional "work hard and retire eventually" mindset is not going to last forever. Today's workers are paying yesterday's pensioners.
A ubi based economy would require more protectionism from govt of finite resources and perhaps even a carrot based incentive system to get people to do less desirable activities. I dont think a ubi economy would have to be entirely flat in terms of wealth, there should always be incentives for people who want more. I'd rather have more sharing of a protected commons than a few oligarchs having it all to themselves.
I don't personally advocate for UBI, but I'll counter your question with another question; how are individuals supposed to have class mobility in an economy where the majority of transactions are speculative? The traditional "work hard and retire eventually" mindset is not going to last forever. Today's workers are paying yesterday's pensioners.