If those migrants were all adjusting well to living in Great Britain and embracing British culture, this would not be a problem:

Britain is set to become the most populous European country by 2050 because of migrant numbers, official figures have revealed. Eurostat, the European Union’s statistical office, predicted the British population to increase by at least 13million – from 64.6 million in 2015 to 77.1 million in 2050.

Those numbers collapse completely if migrants aren’t included, which would cause another set of problems. For instance, the welfare state would become unaffordable. Having said that, it’s not necessarily true that high numbers of migrants and their children will prevent that; that’s only true if they put more into the system than they receive. As anybody who has ever walked around in a large European city can tell you, that’s certainly not always the case.

Additionally, as think tank head Alp Mehmet points out, having such high numbers of immigrants (and their children) in any society can cause very serious problems:

This research spells out the very serious consequences for our society if net migration continues at its present scale, with membership of the single market resulting in a relentless increase in our population. In increase of anything like 12 million in just 25 years is, quite simply, unacceptable to the British public and certainly not what they voted for in the referendum.

Most proponents of Brexit aren’t calling for a complete ban on migration. There are very good reasons why some migration should be allowed — eg., Brits marrying foreigners, highly skilled foreigners who are of value to the British economy, real refugees who can’t be protected in their own region. But that is not the same as having an open-borders policy that allows everybody in who wants to get in. Having such an immigration policy is, quite simply, suicidal.

The good news for Britain is that Prime Minister Theresa May has already promised to put an end to the EU-promoted unlimited immigration. What’s more, once Brexit is complete, Britain will once again be in charge of its own borders, making it relatively simple to keep out unwanted migrants.

Immigration is not necessarily a bad thing. In fact, it is often very useful. Just look at the United States. But unlimited immigration is bad.