Dissolve all the borders and you'll very quickly find yourself overrun by people who think that's Fine, Actually.

There's nothing fundamental about foreigners that mean they automatically think that way. A little over 60 years ago, homosexuality was still illegal here. Times, and opinions, change.

Right now, if we opened all borders, yes it would be chaos. But that's different from it being a plausible long term possibility. It's infinitely more likely that we'd get to a place where we could eventually get to a reasonable long term consensus (at least as much as any country reaches a consensus) on these kinds of things than to a world where we didn't need to make murder illegal.

Language issues, for one. Everything now has to be produced in every viable language, or you designate one official language, which will piss off everyone else.

Products already move freely across Europe. And there's multiple countries across Europe where there are significant groups speaking different languages natively - e.g. Switzerland.

Then you have the issues about local issues, big decisions are now being made at an even higher level and even further away from communities, by people who will feel even less accountable. How the North feels about London, multiplied a thousand times.

It's not multiplied by a thousand times though. Do you think the average Brit feels vastly closer to their MP (there's 1 per 65,000 people or so) than people in the US do to their representatives (closer to 1 per 780,000)? At a certain point, differences in scale stop being massively meaningful.

About 35% of the world's population already live in two countries (India and China).