But, that doesn't mean that you are not free to break up with someone. You just have to be an adult about it and live with the consequences.
In actual abusive relationships this interdependency is one of the important factors that keeps the abused in the relationship. It’s one of the things that makes abusive relationships so hard to escape and consequently so horrific.
That said, interdependency is also just as much a part of non-abusive and completely healthy relationships, i.e. the mere existence of interdependency doesn’t decide whether a relationship is abusive or not.
To me the EU and UK relationship is a clearly non-abusive one. I wouldn’t call it healthy, but mostly because the UK is frequently completely disengaged from the process and disinterested in actually being part of the relationship. They could have so much say and power … but they don’t want to. Rather they want to spend all their time worrying and drawing up horror scenarios.
I do not want the UK to leave, but I also don’t think that a UK this disengaged is not very productive to have inside the EU.
Why is that guaranteed? I'd imagine that the argument can be made in both directions.
Not all of that has to happen (depends on the terms of leaving). I'm not even saying that it's going to last long. But I'd bet on at least 6 months of uncertainty and chaos which is going to affect the economy in a bad way.
Regarding your point about current employees being forced to leave, this is one of the things that worry me most - with friends, colleagues etc. Luckily, most sources indicate that anyone already legally working in Britain would be granted permanent settlement rights. It would be a shame if good people in future are unable to migrate though!