All great objections, and a flat UBI is probably a very dumb idea indeed.
I would recommend a minimum income that is granted based on need and funded with taxes on corporations and the wealthy (who undoubtedly are the ones benefiting from the low wages that make everyone poor to begin with). An amount that is based on inflation and starts as a certain percentage — probably upwards of 100% — of the poverty level might be a good start.
Some of your objections might still apply to this, such as whether we could get people to flip burgers at McDonald's, but from what I can tell, I don't think the world would be in that rough of shape without businesses like McDonald's anyway, so I can't really justify any lamentation of their loss.
For those industries which are essential, we might see the price of their goods/services rising to a level that actually reflects their cost to a better degree. When the price of McDonald's goes up, people might stop buying stuff from McDonald's. When the price of electricity and waste disposal goes up, people might start using less electricity and making less waste, and the negativities associated with those industries would be better internalized and not thrown on the workers (e.g., coal miners who are currently forced to eat the cost of their decreased health and are not properly compensated for what they are really giving up).
One of the reasons stated for a UBI is the next round of the machine age throwing more folks out of work. In reality it makes the robot that much more imperative. A self-fulling prophesy?
So long as capitalism remains the dominant system, there will be no way to mitigate the negative effects of mass mechanization short of government intervention.
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