I do not see this having any lasting negative products.
Frankly, this is spoken by someone who won't ever have to deal with being verbally or physically assaulted on a British street for being an undesirable. Racial intolerance is already growing both in the UK and the US as a result of these movements. So even if the economic fallout is temporary, the nationalist sentiment is a pretty important issue to the people who have to face increased hatred and bigotry.
The economic fallout is minor in comparison -- because most economic slumps do eventually recover. It takes a lot longer for bigotry to filter out of society, and policies like this take the shame out of being prejudiced.
The sentiments were there long before Drumpf or Brexit were things, but now racists and xenophobes feel increasingly justified in their narrow-minded hatred.
Policy-wise, this is the European equivalent of "the US federal government is broken and corrupt -- better off just eliminating as much of it as we can rather than actually find ways to fix it." In either case, it means you're awful at governing. If you eliminate the higher level of government, that doesn't somehow end corruption. It just moves the corruption and stagnation closer to home. European nations and US states are pretty damn fucked up on their own. So are local jurisdictions. For every system humans create, there's always someone who's going to find a way to punch loopholes through it for their own personal profit.
Efficient governing requires constant effort and constant -- and significant -- public involvement. That's where things start going wrong. When the peasants don't educate themselves and assume someone else will fix things. Such is the downside of democratic governments. The average person is dumb and lazy and uninterested in the political process. Unless that can somehow be corrected, inefficiency in government will be an issue on every single regional level.
You can fix all of these problems without having to cut off all your economic ties and throw a tantrum. There's also no guarantee that EU officials will see it as a "punishment" for their own misdeeds.
I note how exquisitely careful you were about avoiding saying things which were untrue. Congratulations, because with the amount of misinformation and disinformation flying around about Brexit, that's actually a mark of significant accomplishment.
That said, I don't think the racial fallout is something we should judge the outcome by. No election result justifies criminal activity. Now, if the UK government decided to stop prosecuting racially motivated crimes because of this--or more likely started selectively prosecuting cases from one group and ignoring those of another--that could become an issue. But that's arguably already a problem elsewhere in the EU--Arab refugees, for instance, chanted Adolf Hitler and Allahu akbar in a protest in Germany, for instance. I've heard German nationals would probably have been put in prison for doing the exact same thing, which, if true, would make that an interesting case of racism. Just because it happened to fall in the xenophilic direction would not change the fact it would still be racism.
Besides, I suspect that a political victory isn't a good motivator for racism. In the South, the KKK went from an old-boys network to a domestic terrorism wing because they lost the Civil War and had no political control whatsoever during reconstruction. They had no political control whatsoever for the better part of a decade, which made every policy change a loss. The losses, in turn, made recruiting easier for a hate group like the KKK because appeals to fear grow stronger after a loss. You would think victories would have a cooling effect, but what I remember of the history is they had little to no effect, one way or the other. I don't see any reason to expect this vote to have generation-long repercussions, although some nuts seem to be falling from the tree during the shakeup.
And yes, I do agree that this is like saying the US needs to be fixed. It does. But the United States is also old and stable enough to swallow it's own problems. It won't make a move to fix itself unless there's a significant competitive pressure to do so. Even insiders trying to force the issue would probably fail because changing big systems is actually really hard. By contrast, the EU is new and Brexit can be turned into an incentive for internal reform. If successful, that could turn into an incentive for the US to reform. Of course, if they don't, more countries will want to leave, and I can't really fault them.
Of course, neither government will like changing. They never do.