The accepted narrative, at least on the left, is that migrants are always and everywhere a net positive benefit. I've doubted that for some time, and the research shows very mixed outcomes. Key variables are the source of migrants and their education. In the case of recent surges to Europe there's the additional hypocrisy by refugee supporters that these waves are indeed refugees, whereas they are about 80% economic migrants. In short queue-jumpers.
This piece, in the Voice of Europe estimates the lifetime cost to Germany alone of the migrants already arrived to be One Trillion euros. They are uneducated and from countries with cultural mindsets inimical to liberal western societies.
Germany accepted 1.5 million migrants since 2015 and Sinn says they are not dentists, lawyers and nuclear scientists, but mostly underqualified immigrants. According to him, these people can never repay what they have received from the German welfare state during their lifetimes.