You get better deals as a group, as a large entity. Yes, I'm sure that deals can still be done, but what deals, we don't know. Europe is by far our largest trading partner, how they would react to us leaving is open to speculation.
I really don't see, or agree how staying is a bigger unknown? We know where we are in Europe. Leaving is pure speculative guess work.
It's not fear. It's more don't try to fix what ain't currently broken. Our economy is doing fine. If we leave it will suffer - perhaps irreparably.
The economy is my major concern. That will take a hit, of that there's no doubt. How big a hit, and how long lasting the damage, nobody knows. And they're a liar if they say they do. If I honestly thought the UK would be better off out of the EU, I'd vote that way. But, I don't. I see no credible evidence that things would be better. Just guess work and baseless optimism.