The idea that immigration is a net benefit is true, especially with an ageing population. However, that does not mean any immigration from anywhere at no matter what pace.
The EU fundamentally broke when they admitted countries which had little in common economically or culturally (including some VERY anti Semitic countries). You cannot have freedom of movement of people and goods with countries which have a GDP of 30% of the richest countries without consequences (and not good ones).
The Euro is also a failed construct for much the same reasons as above. I find it fascinating that so many very clever and educated people who know far more than me are reluctant to accept the historical lessons about currency unions without common taxes, language and culture (none lasted).
I am not looking forward to Brexit per se but having lived through the Asian crisis, the Euro crisis (which was actually pretty big) and 2008 which really was, financially, huge, I don’t think Brexit will be as big a deal as most think, especially given some sort of fudged deal.
If I were to worry about geopolitics, China/US tensions are far scarier, as is the ever increasing amount of Euro debt held by the ECB.
The worst predictions concerning Brexit currently still make us richer in 10 years than we are now. Is that really depressing and terrifying? Of course, you can argue that any poorer is a silly own goal, but there are good arguments on both sides of this.
There will always be things going on in the world which could go badly wrong and, generally, depression about the state of the world is displacement of issues far closer to home.