That said, I expect there are more robust philosophical arguments. I'm no philosopher, but I'll take a stab at it. Consider the prisoner's dilemma. If everyone acts solely in their own self-interest, it's worse for the whole than if people act cooperatively. Human civilization can be seen the same way. An individual might elevate themselves by acting purely out of self-interest, but on the average we're better off acting cooperatively. So I would suggest that not only is there a moral imperative to strive toward equality, doing so will also improve quality of life overall.
And of course, that's natural; people DO value their own well-being and that of their families and friends (and especially children) more than that of strangers. It's a very rare person who's willing to completely sacrifice their own standard of living in the name of global utilitarianism. (Mother Teresa comes to mind, but that's about it...) But it's also pretty rare for a person not to feel any selfless compassion for others either; it's just harder when one feels one's own needs aren't being met.
So maybe that's the real solution to your problem: find a way to address people's needs without resorting to a zero-sum mentality. When people feel they're getting by alright, they will naturally be less begrudging of others. Easier said than done of course, but not impossible. Much more difficult when the focus is on this zero-sum game instead though.
The crux of the issue. I appreciate you taking the time, and employing the insight, to identify it. In my experience of such discussions, few do.
It would seem not, or this conversation wouldn't have played out the way it has. I'd certainly be interested to see a defense of this claim that didn't rely on one's interlocutor sharing one's own moral precepts in order to cohere.
I'm not saying a borderless planet would work out; at least not anytime soon. But we should at least be working towards it.
We know that AI could go bizzarlly wrong, but we still work towards achieving better results everyday.
The point I'm making is that not everyone agrees with you about what's morally right. In fact, most people in the US, at least, do not. If you want to effectively advance your cause, it is worth finding an argument for your claims which does not rely on moral precepts your interlocutor may not share.
As a side note, likening the existence of national borders to the institution of slavery is probably not such an argument.
It's not a surprise to me that it's most people in the US that are against the idea of this, because they fear that instead of the datum of other countries moving up, that of USA will go down. This insecurity always annoys me. But take everything I say with a grain of salt because I do not consider myself patriotic whatsoever, and in that sense, my thoughts are in the opposite direction as those of Americans, who, through personal observation are the most patriotic people I've come across.
I agree with you that while there are some appeals to self-interest that support a goal of equality, fundamentally it is an issue of morality. And I agree with you that you can't logic someone from one moral framework to another. But I would add that almost everyone does have some amount of 'maximize the greater good' built into their moral framework already, so the only logical way to work toward this is to focus on the variables that will add weight to that side of the equation.