If these migrants were the educated middle classes of the 3rd World then yes they could probably contribute positively. But Sweden, Germany and many others have no standards. Anybody can come and say "Asylum" and they stay. There are 40 year old men claiming to be 15 years old and instead of giving them a good kick in the ass and putting them out for lying they are put in schools with real 15 year olds. Few people are anti migration, what people want is standards. Let's call it quality migration. Nobody is against a A grade University student from Ghana coming to Europe, but do we need his uncle who is a unskilled labourer?
Unskilled Europeans have trouble contributing positively so it is illogical to think that unskilled foreigners who don't speak the language can do this.
It is a fair question.