Policy is decided, implemented and evaluated on the level of societies, not on a global level. We have no global government, and the only global cooperation is voluntary, with wealth redistribution only happening as much as one society arbitrarily chooses to spend on charity and foreign aid for another. Social policies are not implemented worldwide but rather by some society for its own benefit, and if some policy makes a society successful, that's a great policy for that society. Leaders in democratic societies are accountable to people of their society about ensuring that their needs, desires and choices get met, they are not accountable to humanity in general but rather the people they represent, and in general these voters give a very limited mandate to sacrifice prosperity of their society to "raise the bar" of worldwide prosperity. If the society chooses to make policies that benefit everyone else, great, but if it does not, then the policy is good if and only if it fits the choices of that particular society. We can make a strong argument that there is a moral imperative not to harm other societies, however, ignoring them is an acceptable 'default' state of noninterference.
If doing something allows you to "pick winners" so that all of your society are winners, that would be great (certainly better than what we have now) and would have effectively solved things that matter for that society, even if it does not extend to the world beyond its borders.
To wit: https://www.webfx.com/blog/internet/the-6-companies-that-own...
The Republic model (not the one described by Plato) is the best solution we’ve found to date to deal with these flaws.
- If you don't allow foreigners, I'll argue that you haven't solved anything. You've simple picked the winners.
- If the ratio of inequality is disrupted, I'd also argue you haven't done anything other than move the problem (and the losers) to the future.
- If you decide not to redistribute, well that's the current situation for better or worse.
Personally I think a better problem to solve is for humanity in general to decide what the minimum level of prosperity should be, worldwide and slowly try to raise this bar. AFAIK this is basically what we're doing. We could raise the bar faster, though.