[dragonwriter]

The problem you point to is self-limiting. Regardless of the nominal level of UBI, it won't provide a satisfactory income unless a sufficient number of people remain working to provide the goods and services required to provide a satisfying style of life for the UBI-only recipients and sufficient additional value to workers for them to continue working. Prices will adjust to make this the case.

As the quality of life provided on UBI-only declines, the incentive for additional productive work will increase.

(of course, lots of people now continue to expend additional effort beyond what is necessary for a minimally livable existence, so I don't see this as likely to be a great problem.)

[hahooooo]

In other words, the tragedy of the commons.

My biggest concern with the UBI is that looking back at societies with UBI (such as the upper class Europe of the Middle Ages through the 19th century), for every Newton or Voltaire were many more unknown nobles who spent their life grooming their mustaches.

I don't believe our society is that much better now.

[dragonwriter]

> In other words, the tragedy of the commons.

In a sense, but in this particularly case it operates as a negative feedback control mechanism which prevents the actual feared problem from occurring, so is hardly a "tragedy".

> My biggest concern with the UBI is that looking back at societies with UBI

There are no societies with UBI to look back on.

> such as the upper class Europe of the Middle Ages through the 19th century

A benefit restricted to a particular social class is not a UBI (or, at least, not much like what modern UBI proposals target, and not likely to serve as a guide as to what one can expect from them), nor, even ignoring that aspect, did the upper class of Europe through any of that time have anything remotely resembling a UBI, which is a flat, equal, unconditional grant to all members of a society.

Now, in much of that period, individual nobles had (either by definition or just disproportionately, depending on the particular time and place) either claims to income from substantial properties or family connections to those who did and who would support junior family members with, often quite conditional, support, but that's more the position of a wealthy capitalist or their associates in the modern era than that of a UBI beneficiary.

[noonespecial]

We'd have to decide how many mustache groomers each Voltaire or Newton is worth before deciding if it's a tragedy, no?