Purely moral concerns
Separating the concerns of earning and retention1 is a nice way to argue for the morality of riches. The point is that whether you earn and deserve earning much money and whether you should retain it after the fact, are different and separate moral questions.
Thus, retaining wealth is imoral. Earning it is not.
There are obvious logical pitfalls that one can fall into by following these arguments too deeply. This sort of reductionism nullifies the applicability of the principles that could otherwise arise from a considerations like this.
As a potential solution A. Q. Smith suggests setting down a limit for a “maximum moral income”. Increasing the limit fore each child a person has is suggested as well. This would not be the only possible solution and he mentions several others in the final paragraph.
The point being that there is a limit and that everything above this limit should be given away as it is immoral to keep it for oneself given that people around you are suffering. Not doing so is akin to not intervening when you see suffering first hand, and you have the chance and possibility to do so!
In the final paragraph Smith1 very precisely spells out:
The central point, however, is this: it is not justifiable to retain vast wealth. This is because that wealth has the potential to help people who are suffering, and by not helping them you are letting them suffer. It does not make a difference whether you earned the vast wealth. The point is that you have it. And whether or not we should raise the tax rates, or cap CEO pay, or rearrange the economic system, we should all be able to acknowledge, before we discuss anything else, that it is immoral to be rich. That much is clear.
Ecological concerns - deteriation of the commons
Smith’s suggestes solution of a “maximum moral income” 1, makes me think of a similar consideration already implemented in the SDI index.2. Here average per capita incomes above $20k a year yields no further index score gains.
In the HDI, an income of $75,000 per capita is set as the cap. The reason this is lowered in the SDI is becaus it is considered that:2
This is because to achieve an income of $75,000 per capita is empirically incompatible with planetary boundaries.
It’s thus quite easy to draw up an approximate parallel between the two arguments, the difference being on which grounds the immorality is posed and the direction of attack:
For the SDI the consideration is too high resource throughput: you can avoid “ecological suffering” by not earning/spending the money.
Smith argues more generally, the main consideration being general human injustice: you can directly alleviate human suffering by giving away everything above a certain limit.
As I see these considerations are not necessarily incompatible.