The main flaw is that one of the concerns with UBI is that many people might never start working and by the time they realize that UBI isn't enough, it may be too late. OTOH, someone who has been in the military for 20 years may well decide to retire early but that's a different situation.
So implementing something so drastic without knowing if it'll work is extremely risky.
Would you say that every country on the planet is either "capitalism with some social features built in" or is currently failing hard? And if all those systems are capitalistic, what specifically pushes UBI over the edge into being something that isn't capitalism? From an implementation perspective, UBI is simply an adjustment of the income tax rate including allowing that rate to be negative. It has nothing to do with ownership, markets, or many of the other mainstays of capitalism.
Yes, if by the overcomplicated first description you mean modern mixed economies (which are more accurately descended from capitalism than actual capitalism), and those groups overlap, too.
> And if all those systems are capitalistic, what specifically pushes UBI over the edge into being something that isn't capitalism?
Nothing, it's well-within the broad outline of the modern mixed economy.
> From an implementation perspective, UBI is simply an adjustment of the income tax rate including allowing that rate to be negative. It has nothing to do with ownership, markets, or many of the other mainstays of capitalism.
I think it's debatable whether downward redistributive taxation/spending does or does not conflict with mainstays of capitalism (I think it doesz in a good way), but in any case that's a feature of modern mixed economies generally, and UBI isn't really a difference in kind. Where UBI perhaps is more at odds with capitalism than traditional mixed economy welfare programs is in weakening the divide between the chronically excluded and the working class (the lumpenproletariat and the proletariat, to use the Marxist terms), which I think is arguably an important feature of capitalism that is underappreciated specifically because Marxist-inspired socialists don't usually directly address it, especially in a transitional rather than revolutionary concept, because they viee empowering the lumpenproletariat as dangerous because they view that class as prone to recruitment as paid agents, though not actual class allies, of the bourgeoisie while not being effectively organizable against the bourgeoisie.
The current social safety nets are not as drastic as something like a $1400/mon stipend for everyone in the country (as Germany is trying). That's $5 trillion a year in the US. That over doubles spending right there.
In 2019 the US federal government had $3.5 trillion in revenues and $4.4 trillion in spending. Add another $5 trillion -> $9.4 trillion in spending with $3.5 trillion in revenues. We'd have to almost triple our revenues!
I know there are plans to pay for it, and it's probably possible. But you can't deny that it is drastic.
It's very dangerous to implement such drastic changes to the economy. And the consequences of getting it wrong are horrific.
Testing in production (with rollbacks and safeguards) is actually how things are done in many endeavors in life, especially with processes that are reversible or where the cost of mistakes are bounded. Reality is a better teacher than a simulation. This it not to put down tests -- it has its place -- but in complex systems, it's impossible to have comprehensive coverage. Stochastic things show up in real life that never show up in tests.
In software, you just keep building up your test suite. In complex stochastic systems, you'll have a much harder time building tests.
Software people have a notion that the world should be TDD-driven, but it doesn't really work like that.