If you want ethnonationalism to finally go away, your best bet is to put a halt to inward migration. Someone who has a neurotype like ethnonationalists knows how they think; as long as it's aliens moving inward, they have the timeless classical narrative of the homeland valiantly defending itself against looters to keep their cause powerful. Take that argument away from them by sealing the walls, and the next
item on their agenda, the separation and expulsion of peoples who have been on the homeland all their lives,
will be laid bare, and the general public will see the pettiness of their endgame desires and shun them once more. Take that narrative away and ethnat will have nothing left to stand on. People don't give a shit about statistics or what others look like, they care about the danger of being obsolesced. They won't have to default to the extremes to save them if the original danger is gone. Neo-nazis were only the anathema in the public eye until this migration situation became a thing and suddenly they went from the gross fringes to a cult fascination and eventually a viable choice.
"Abolish all nations" is great when you have the tools to abolish the independently wealthy as well. But we don't.