>It's interesting, why do you think individual states in the US "giving up their sovereignty" by way of control on migration was successful, but doing so at any broader scale wouldn't be? That's 325M people -- they're not all in California and New York, are they?

For one, who said it was successful? As you say, those US states lost their sovereignty in the process - which is exactly what I maintained nation states would probably want to maintain.

Second, unlike existing nations, those states were "tabula rasa" from a cultural perspective, huge empty areas to be filled with people (although even in this case, things didn't ended very well for the actual natives, the native Americans).

Third, it's not like those states have much of a meaningful local culture, or much on taxation and other such policies. Thank's to gerrymandering and a broken party people they can even get a president most of their people voted against. And the coasts have to suffer the politics and politicians voted by the mainland states, and vice versa.

Surely they are not the best example to make the case for "open borders while maintaining one's sovereignity".

But I guess your question is not predicated on that, but on the migration patterns to those states. As in, people didn't just jump in there from all areas -- e.g. to all end up living in California or New York and so.

But that's also a broken analogy. If NY had a much better standard of living than the other states, so much so that people were willing to leave their own states, local communities and connections, people would immigrate there in droves.

Lots did actually - e.g. millions have immigrated internally into California during the Dust Bowl crisis, creating shanty towns and lots of turmoil in the process. But while there is still work to be found around most of the 50 states, there's no much reason to predominantly prefer one over the other. And the US has been relatively stable and prosperous economy, in fact the #1 economy in the world for a century or so.

Here we're talking a billion (or more) people from developing countries that wouldn't mind to make the jump to another western country.

>Moreover, why doesn't everyone in the free movement area in Europe pile into Switzerland?

For one, because the difference in quality of life is not enough to leave e.g. Sweden or Spain, or France, or even Hungary, and go to live in very expensive country, without a job waiting for you.

You are right that "Plenty of people move from eastern Europe, but there's still last I counted 38M people in Poland. They're not all in the UK, Germany or France. Local language, local culture, local jobs, economy, opportunities, family -- these all tie you to a place." -- but we're not talking about people with "local jobs, economy, and opportunities", but about hundreds of millions in much poorer regions, that start from much lower conditions.

Second, because you need to obtain a residency permit, and prove that you can sustain yourself while in Switzerland for any stay over a few months. So willy nilly migrating there without money in the bank, being accepted into a university, or a job waiting for you is not an option. And not just that, but the Swiss have voted to restore immigration quotas for EU citizens (and already have special stricter quotas for specific countries in EU, namely the poorer ones).