The downside to this is that (for the most part) a basic income mucks with the incentive structure.

"Mucks with" is not a positive term. And to be sure, there are employers for whom the introduction of UBI would be a very negative thing. But there are people (many more, I suspect) who would see UBI as a positive development. These people would not say "it mucks with the incentive structure". They would say "it improves the incentive structure by making it more humane".

Why? Because if you were an employer with a business model that relied on easy access to the bottom end of the labor market, abject fear would cease to be a viable management tool. In a world with lower barriers to quitting you'd no longer be able to pad your margins by exploiting workers in a way that would be inconceivable if they didn't live in constant terror of hunger and homelessness, should they suffer any disruptions in their jobs. To put it more politely, as a manager, you'd have to make more use of carrots and rely less on sticks. This would certainly muck up the game of anyone who relied on nothing but sticks, but again, that would be an improvement in the eyes of many.

Consider, for example, the employers that refuse to give people full-time work to avoid paying benefits while simultaneously refusing to give people regular schedules, which bars them from taking seconds jobs. These employers are demanding full time availability for part time returns, and getting the on-call time for free. This is atrocious. While I can only speculate here, I suspect this practice would be far less common in a world where people desperate enough to put up with this abuse were significantly fewer and further between.

What opponents of UBI must concede is that the end of labor extracted on highly unfavorable terms from people who fear for their very survival does not mean the end of labor in general. As most people on HN know, there are many reasons besides basic survival that motivate people to expend effort. Taking some of the negative power out of employer's hands would re-focus their efforts on developing non-abusive ways to keep people expending effort on their behalf.

That's bad news for managers who lack the ethics to be managers in the first place. It's good news for decent managers who no longer find themselves lumped together with thugs and thieves (and yes, wage-theft is endemic at the lower rungs of the economy).

I should point out, too, that only a fraction of the work that people do is also paid employment. Any mother who works full time raising her kids can attest to this. The same could be said of working out regularly. UBI acknowledges that there are all sorts of socially beneficial activities that require considerable effort, but don't require formal employment by staffed organizations.

At present, when we say "work" what we really mean "work done directly for someone else in exchange for money". It's important to note that while UBI may, indeed, reduce incentives to work directly for terrible people who pay a pittance, it doesn't mean that people will stop working for the good of themselves or others in general. Indeed, given our intrinsically social nature, freeing people from the monsters may lead to a spike in the output of socially beneficial effort.