Because the market value of just being average in what you can do could be below what we deem tolerable.

It does depend a lot on what's tolerable, admittedly. As long as we reason about it knowing that wether we like it or not, many of these people living in less than desirable circumstances will have kids and those kids have never done anything to effect wether or not their situation is tolerable.

I'm not sure macroeconomic systems affect the kids as much as socioeconomic ones like household mobility, addiction rates, mental health care, and family structure.

Yes, but then we're incentivizing the future supply of such "below average" individuals that you speak of. That in turn reduces their value by making it a race to the bottom. Which of course, causes our caring society to kick into even higher gear in terms of "caring" for them. And so the cycle continues, with the bottom half eventually tipping the scale immensely. That is why you're getting "inequality", because you're subsidizing its increase without fixing any of the underlying problems.

Edit. Grammar.

A society claiming the unwealthy tolerate their outcomes because of its exceptional fairness has to prove that unwealth arises from a person deciding to abuse their own chance at wealth by more than the fair treatment society entitles them to (because those ending up wealthy were ostensibly assumed to be given no unfair advantage).

If fair treatment from society (for the purpose of equal opportunity to wealth) results in unequal but 'consistently fair' wealth outcomes, then the explanation for the unwealthy individual's outcome must be disavowal of wealth "after being given the same chance everyone else had" Which implies that, since fair treatment failed to ensure the goal of wealth was as reasonable for them as anyone else, then the sole remaining factor is supposed individual disavowal - which tautologically represents an intolerable situation after all..

That's the sort of low-quality insult that isn't really welcome here, but I'll dignify it by pointing out that Warren Buffet is among those calling for higher taxation of rich people.

Just look at history, or current conditions in some parts of asia: child labor, 16-hour days in overcrowded fire hazards etc.

But the more general point is: do you value an individual solely by what they can produce?

Extreme example: Late at night, Mr.&Mrs. Miller get in a car accident and die. Their 5-years old daughter survives. She is not capable of sustaining herself with productive work. Is it fair for her to die?

Yes, it's extreme. In any sane world, someone would somehow take the girl in and provide for her until she's old enough.

But taking in an orphan and caring for them is already altruistic. Continue down that path, you'll soon stumble upon a social safety net that canonises these instincts. That's better than individual initiative because:

- it can distribute the costs of caring equally, instead of burdening, say, one family with all the costs of raising little Amy Smith

- it's fairer: your survival should not depend on you being cuter than someone else in need

- it's dependable: there's value in knowing your child will be cared for should you suddenly be hit by a bus