I would assume the burden of proof to be on you (and supporters of UBI) to prove that opposite will happen and people won't just slack off once their basic necessities are covered.
Most people who have the opportunity now don't slack off once their basic needs are covered, though the marginal effort needed to go beyond that is greater than it would be with a UBI. It is UBI opponents who are positing something contrary to all human experience in arguing that people in general would stop seeking improvement in material condition if the marginal cost of that beyond basic survival were reduced by UBI, so I think the burden is on them to, at a minimum, a rationale for believing that to be the case.
Though dealing with concerns like this (which, rational or not, are clearly significant) is among the many reasons why I think a gradual phase-in of universal income, gradually phasing out means-tested safety net programs, is better than trying a radical transition.
So again, can you please provide objective evidence for your assertions? Eg. "vast majority remains productive in some capacity." - I have no idea whether its true or not. Why should I trust you when you haven't provided any data to support that claim?
This is only true if the existing system does not keep on worsening. As the technology progresses human working hours has become more and more. How is this even fair when the whole point of technology is to make human life easier. So the Burden is on the people who want the current system not to change.
What if it is impossible to bring the proof before the change has been tried? There is essentially no test scenario which can replicate, accurately simulate and predict the behaviour of people when millions of them suddenly receive a UBI.
At least if one agrees with the claim I just made, then following your logic, the change could never happen, for the simple reason as advanced proof of the effects is impossible to provide. So you would simply never find out if the proposed system would be better or worse than the current one.
Having said that, I do think the article makes a point worth considering.
... big changes could never happen... FTFY!
Which is what we are seeing: non-trivial health care changes happened in 2010 after 1965 (45 year gap), needed massive majorities in Congress and incurred huge political backlash. i.e. a smaller step was possible only after all the stars perfectly aligned.
Instead of volleying opinions back and forth, I think we would do better by discussing the ins and outs of current problems to which UBI is a single proposed possible solution.
How many well-off people that have their basic necessities covered just slack off?
Well to some level we can handle that right. If taxis are driving themselves, and other jobs get replaced by tech havent we liberated these folks?
If people want to work to feel useful great, but the point of technological advancement is that not everyone will have to.