Thank you for shining late to some Economics basics. According to what you are pointing out, then, UBI is not a matter of affordability. It could be a stimulous-investment on behalf of the government, just like investing in infrastructure would be.
Furthermore, if what balances out the ‘debt’ is taxes, UBI has the potential to increase spending, therefore increase tax-intakes. Now whether input from taxes would balance out govenrmental ‘spending’ on UBI… I don’t know. What do you think?