Some observations from a Yank.
1. The attitudes presented to the public here in the states was decidedly one-sided. Journalists, politicians, corporations all sang the same song. I found their remarks hypocritical and selfish. Selfish for the simple reason that Brexit was presented as a threat to our finances. Hypocritical because no one in their right mind (on this side of the pond) would ever imagine allowing other countries to, at least some of the time, dictate how we do things? We can't stand it when someone across a state line, or the Mason-Dixon line, or a chalk line attempts to decide things for us. So you Brits must do what we would never do.
2. As mentioned, regardless who won there was going to be millions of Brits upset about the outcome. What those of us who live elsewhere need to keep in mind is this: half the population of GB wouldn't have felt the way they did if the GB-EU relationship was all rosy. Maybe the differences could have been worked out in time, maybe the differences can still be worked out, but I don't think you can fool half the population. There were many legitimate problems between the two and evidently those who voted to leave weren't convinced those problems were going away any time soon.
3. Hyperbole. There is an abundance of the stuff floating around these days. "Devastate" probably fits the definition in this case. I personally think GB is more than capable to manage its foreseeable future. But it helps no one to express the pain of defeat by trying to bludgeon someone with doomsday talk.