I think the author probably agrees with you that London won't "fall" in the sense that it will cease to exist, or even fail to prosper. Rather, she's arguing th

I think the author probably agrees with you that London won't "fall" in the sense that it will cease to exist, or even fail to prosper. Rather, she's arguing that London is the most important city in the world due to its connectedness to Europe, and as a result of Brexit it will fall behind other cities and lose its position as the most alpha-est world city that there is. Not catastrophic by any means, but if someone is living in London because they consider it to be the center of the world, then it's a big deal.