UBI can only succeed if people continue to consume, preferably at higher levels than currently, which would allow others who want to live a life at a higher standard to sell things to UBI recipients, who can then be taxed and support everyone else on UBI.
If UBI encouraged too large a portion of the population to pursue non-taxable income (say, making art and games and giving them away for free because they don't need money), then UBI would collapse from lack of income. Especially so if that art is given away freely to other countries over the internet. So it's a fine line that is probably a lot more complex than people give it credit. There's certainly no guarantee as to what the results of UBI would be, and no guarantee that the medicine won't be far worse than the symptoms.
EDIT: Even if you assume that a UBI could survive if it decreased with a fall in taxable income from a policy and livability perspective, if a person on UBI received $100 one week, and then had it drop to $75 the next, he would get a major shock and public opinion of a UBI would fall apart. You could print money to make sure UBI never falls, but if UBI is going out to the entire population, the amount of money needed would be enormous. That much money added to a system without an increase in real demand (people on UBI would still buy the same stuff), could easily cause hyperinflation. So another constraint that UBI has and I've never heard anybody mention - UBI can never decrease, even if tax income decreases. Plus there's no way anybody could trust politicians to manage something that complex. UBI practically demands a complete restructure of government, society and the economy. Instead of being a 'patch' as many have described, UBI is closer to 'The World 2.0'. It would be the most incredible Second System Syndrome ever seen.