If rich countries' sustanibility depends on poor countries staying poor, it creates a huge incentive for the rich countries to destablize the poor countries and keep them poor.

Just like the US once destablized its southern neighbors to keep them exporting cheap fruits, if the only thing that keeps the US's pension system from exploding is cheap workers from the neighbors, it'd want them to keep exporting cheap labor.

Of course one might argue rich countries will do that anyway so it's not a concern. It's just icing on a poisoned cake.