What countries would want to leave the EU? I'm not aware of any of the poorer countries wanting to leave. And it would not affect the EU much. For the richer countries, the EU is an enormous benefit, so leaving would be rather silly.
The UK is the only country that has been questioning its EU membership for many decades. But it seems to be unique in the position.
So for normal trade in goods and services, it doesn't make sense for the EU to impose too many barriers.
The financial sector is an exception. It does make sense for the EU to grab more power there.
The big issue however, is that many countries have significant parts of their population that are against the EU. So there is a real risk that the UK will be hit with sanctions, not because it makes sense for the EU, but because it makes political senses in some of the EU member countries.
That would be sad, but that's the world we live in.
Do you know what you're talking about? That's a sincere question. My understanding is that the UK's significant trading partners are members of the EU, and that it's the EU, not the individual partners, who will have to decide on any new trade deals with the UK. As others have said, what sort of precedent would the EU be setting if they gave a withdrawing member the same benefits it had as an actual member? The EU does not want more members withdrawing.