Original post by SouthwesternSelective immigration is theoretically-good, but doesn't work. I'll give you a case study from my hometown of London.
My friend is the descendant of EU immigrants. His well-educated parents both work in sectors that are struggling to find UK natives of decent-enough qualifications to fill the job vacancies within it - such as nursing. The NHS has been accused by South Mediterranean governments of 'violently' campaigning for nurses from those countries to come to the UK because not enough British people want to become nurses given the long training period and the poor pay - so the NHS runs huge campaigns in South Mediterranean countries to encourage nurses from there to immigrate into the UK, and even though they earn less than £35,000, it's still more than their native countries can afford to pay them. Immigrant EU nurses therefore help to alleviate the NHS crisis in the UK.
When my friend was born, his parents didn't know how to handle the early years childcare. They didn't feel comfortable with delegating the childcare to special minders, especially as they didn't particularly want a second child, nor were they comfortable with leaving him with family friends during his childhood - not that his parents, being EU immigrant city workers, had many family friends they could ask. Rather than moving back to their EU country, where they could actually delegate the role to aunties or my grandparents, my friend's grandparents came over to the UK from their EU country.
Now, if selective immigration had taken place, they wouldn't have been able to do that. As a result, my friend and his parents would have returned to their EU country of origin - meaning the UK would have lost out of having two skilled workers and my friend wouldn't have grown up in the UK, so he wouldn't have worked in the UK as an adult and then contributed positively himself to the UK economy like his EU immigrant parents. Fortunately, because his grandparents chose to came over, they could all stay in the UK - and although the grandparents don't work and do use the NHS, and although my friend is being educated in a UK school - the net contribution to the UK economy of that family as a whole is positive because of the high-skilled work of their parents, and the fact that all of the family itself needs basic resources (like food and clothes) which in turn increase domestic demand for goods which in turn fuels supply and jobs. By the way, you don't need to have any skills or qualifications to contribute to economic growth through the latter way - you just need some kind of income to keep on buying clothes and goods in your country of residency.
This is one story to show how selective immigration doesn't work and often backfires. You say 50,000 well-skilled immigrants are better than 300,000 unskilled immigrants - but as you can see, sometimes, you need both kinds of immigrants if there is to be any benefit at all. The moment that you enact selective immigration, you inadvertently discourage a lot of positive immigration to your country - because individuals won't be able to immigrate freely into your country - and you therefore have, in the case of EU immigration, a less skilled and less educated workforce.
There are various studies which allegedly prove that selective immigration always backfires in this way. I personally haven't seen them, but nor have I seen any other studies arguing the opposite. In my view, however, it's common sense. Yes, we want to encourage positive immigration to the UK - but it's not pragmatic to label every immigrant as 'positive' or 'negative', because when you get rid of some so-called 'negatives', you will always get rid of some 'positives' too who depend on the 'negatives' being there. Immigration should therefore never be judged on individual bases and always on whole bases - and in terms of the EU, that means recognising that EU immigration is on-the-whole positive, and therefore that we should celebrate it and not demean it, as the 'Leave' campaign is doing.