The first leak in the dam doesn't look so threatening...
There are economic ties, political, cultural, and military. It was always complicated with EU. Some countries are part of NATO, some aren't. EU doesn't have a military. So geo-politically it can't "project power" too much as a unified force. US and its NATO members know it.
If you read foreign affairs journal (they are stinktank propaganda often but they are read and acted on by many in power). You'd see articles about how EU is a geopolitical threat to US. It would eventually want to extract and use the same resources. It would want to eat the same bananas and dictate policy in parts of the world.
EU was and is a threat to US. If anything before those countries formed a stronger union, they were easier to manipulate as they could be played against each other. Like say offer Italy some trade rights to force France to change their stance or something like that.
Kosovo war for example was a good ol NATO show of force. USSR and then Russia was disabled in the gutter licking its wounds, EU was rising. It was time to bring in NATO to show the world who really rules Western Europe lest EU gets too cocky or gets some funny ideas.
Culturally, well we are friends. Americans love to trace their ancestry. Yes even 5 generations back and claim they are from Italy or France or what have you. They don't speak the language but sure as heck talk about how Italian they are. And so on. At least as far as culture and shared values. There is a closer relationship.
Anyway, I am just rambling without many references but just trying to say that it isn't that simple as "I thought we were friends and now look at what you guys did?"
These perceptions may be entirely factitious; but they are substantive in that they are shaping the political discourse sub rosa across the EU.
If the UK loses its continental influence, the willingness of Euro-based business to respect the Pound (in trade, I mean - their acceptance, online or elsewhere, of the Pound as currency without first converting to Euro) may diminish. Further, London may fall out of the tech industry's graces as a good destination for regional headquarters.
In either case, the UK's loss is France/Germany's gain - likely moreso Germany's because they're already ahead by a bit.
Being an American, I know less than Jon Snow about how EU power works, but if the UK's losses mount and create a power vaccuum at/near the top, Germany and France are leading states even moreso than they were before. What that might amount to, I couldn't say.
This is cheating on my part, but I agree and disagree.
a) Yes, it's unlikely any one event has the necessary inertia to shift a continent's finance capital from one city to another.
b) Everything starts somewhere. No single assassination is likely to start the largest global conflict in human history, yet Franz Ferdinand's assassination is credited with having "started" WWI. In reality, a number of necessary elements contributed, but human history is written in narrative fashion, and scene 1 of WWI begins with Franz, the way most people tell it. Similarly, the last ~40 years have been very interesting, currency-wise, I think, so, it may do to have an eye out for something big enough to light this powderkeg. Since so much of finance is based on trust, in one form or another, an event that dramatically reduces global trust in the dominant parties of the era would be one logically viable catalyst.