A free migration world helps bring the majority of the population to the same income and economic level and while this is great for africans, middle eastern, eastern european, asians, south american etc it's terrible for the majority of population in england germany france etc... Some wealth gets created when you get more consumers but not enough to increase the standard of living for everyone (most of the time it's just for the land/production/patent owners). If you work a not very high skilled job in the rich countries and suddenly there are imigrants that would do the same job you do for less pay less benefits less vacation less whatever the state allows the business owners to get away with they will choose the imigrants because that's capitalism. Of course the guys that are in position to lose their jobs fucking hate it. Most of them don't have the intelect or training to be computer programmers or be pretty ladies in PR marketing or HR so they get upset. That's the economic aspect of migration.
Yes, however, we're talking about EU migration, in EU countries, where there are certain standards to meet in terms of pay, sick leave, holiday pay etc. I know this isn't always the case and people find ways around it, but in general migrants are employed and get the same rights as anyone else.
You also leave out the fact that migrants bring about more jobs, new businesses, organisations etc, that employ others in their host country. The statistics in the UK show that Migrants paid more into the UK economy that we give out.
Although I understand people's reasons for not wanting more migration, I feel much of it is negative hype towards it. The NHS in this country would literally crumble if it weren't for trained nurses and doctors coming from outside the UK. Not to mention the many menial jobs that often people living in this country wouldn't look at and certainly wouldn't do.
As well as that, the way we connect, travel, move around the globe has become so much easier, and I say this for anyone holding a western passport, and the privileges that comes with it when travelling or living abroad. If we expect the right and ability to go and live somewhere else and contribute to another country's economy, and we want to travel there for our holidays for our adventures and little get aways, then surely we should expect that people will want to do the same?
Then there is the cultural/religious aspect. While the affluent countries population has been trained over decades if not centuries to accept different people and cultures (most of these countries do have a colonial history that brought them into contact with other cultures importing and integrating the best aspects of these cultures while still holding the upper hand). The new wave of imigrants mostly the muslim ones simply refuse to integrate they do not become part of the culture or do so very slowly which is both hard and easy to understand. It's hard to understand in a world so connected by the internet with access to everything and everyone that you find it difficult to believe how people are so intolerant of others. Yet if you really think about it in a world with less connectivity if you were migrating somewhere you usually lost your access to your old culture and would have had to adapt and integrate if you wanted to make it through. Technology allows us to live in our own bubbles. Believe in flat earth there's a bunch of other idiots around who believe in the same things you do a few clicks away. Believe you will go to heaven and get 77 virgins (mass produced somehow) if you blow up your neighbours just get on the internet and you'll get training and support for your crazy shit. This makes integrating these peoples a fuckton harder than before all this internet thingie. People that do not integrate that have fundamentally different values and goals cannot form a working country.
again, Brexit isn't about religious people, or Muslims. It's about our relationship with the other European countries, which last I looked aren't Muslim.
I also don't think many people actually integrate into host countries they end up living in, but with certain groups of people, there is an expectation for them to do so. Go check out many Brits living abroad, who are technically migrants, yet we refer to them as ex pats, who often don't learn the mother tongue of the country they are in, who open up or locals end up opening up British style businesses to accommodate them, where many will group up in certain areas of the world where other Brits have made a new life for themselves etc etc. There's no expectation there for integration, yet it's a must for other groups within Europe.
we do live in a bubble, whats scary though is that our total focus is on this bad Muslim man who might come blow us up, when most of the extremist activities in the UK at least, if not across Europe is perpetrated by white far right extremists, and you're more likely to be killed by a white man than some brown skinned Muslim man. That is very much part of the bubble that caused Brexit, because actually the people voting to remain have no idea why anyone would vote to leave and those voting to leave are filled with all these fears around these 'other' people coming into the country and destroying it
Poorer people also usually tend to have a lot more children than richer people especially those that are religious. Economically speaking this is a good thing since western countries are all suffering due to aging populations and real problems with pension systems (many of them being treated as money to be invested rather than you know savings). However this replaces the current population with one that has different values that has a real chance to vote in democratically laws that are completely non democractic by current standards.
Again, I think you miss the point that Brexit is about European migrants. It really made me sad when I got calls from friends who are academics, who work in our NHS, professionals from Spain, Italy, Poland, Greece and other countries, who paid to study in this country and remained to work. White, Christian, Europeans who now feel completely marginalised by a disgusting debate that revolved around anti migrant rhetoric.
But on your point about poorer people coming to Europe and values etc, I actually don't agree. I agree that many will have more children, at least first generation migrants, I just dislike the notion that somehow our values differ so greatly from people who come from third world countries. What are the values? if democracy is one, then I can assure you that generally people leave undemocratic countries because they want democracy and politicians who at least you get some access to and some freedom of expression. One of the things which has always made me laugh is the whole British values stuff, that no one can actually set out what they and why they're any different to anyone elses values.
This is of course meaningless as all jobs will be replaced by robots and Google Skynet will kill turn us all into bateries for the machines.

I wonder what values they'll have:P