Doesn't that always happen step by step, with "early adopters" arguing on grounds of morality and other arguments, until eventually enough people start to believe something has a high moral value and should be raised to the level of a human right, enshrined in law?
Those who would argue for open borders as a human right are like "early adopters" of that idea.
We would all fit easily on Iceland, but most would die of dehydration or starve in a short amount of time.
I could be an early adopter of "cheese for everyone", but I would need to get a lot of cheese first to make that happen.
That said a world wide safety net would be a good thing to advocate for on moral grounds, no?
The EU has open borders for all EU citizens, and for the most part it's faring ok. There has not been a Europe-wide social safety net crisis as a result of this freedom, and crucially for this argument: there isn't an EU wide social safety net. It has freedom of movement without a universal safety net.
The USA's states are effectively countries. All USA citizens have open borders to travel between states in the USA. For the most part, it's faring ok and there has not been a USA-wide social safety net crisis caused by this freedom either.
Iceland and cheese for everyone: That sounds like what's called the slippery slope fallacy. https://en.wikipedia.org/wiki/Slippery_slope
That was set with our Constitution, and thoroughly tested in the 1860s. The idea has never really been seriously tested again since.
It’s a quite different setup from the EU.