Yet, given history, I doubt that the median-minus-one politician could have the courage to end the current welfare programs if and when a UBI system is enacted. So I'm ... ambivalent ... about what to think of UBI.
You can do this in a way that avoids forcing politicians to have the will to eliminate existing means tested programs but achieve that in effect by simply, add party of the phase in, specifying that UBI income counts as income in applying benefit formulas for existing programs.
It'll be much easier to kill each of the programs when no one has a low enough income to qualify for it.
You don't honestly think that those of us who have doubts about UBI are actually cackling and rubbing our hands together gleefully over all the human misery we're causing by denying living income to the miserable, do you? Of course that would be nice - and I do believe that humanity will eventually get to that point, but we're not there yet. We'll have flying, self-driving cars long before we have the level of automation we need to let everybody kick back and pursue whatever intellectual interests they find.
UBI implemented as a negative income tax would result in much less waste and more freedom to the beneficiaries. It's also politically viable as it could be seen as more "fair" since technically everyone receives it.
Edit: I think the "automation displacing human jobs" argument is easily the weakest in favor of implementing a UBI, especially since it's not necessarily economically realistic.