> I'm skeptical that's a significant factor, because right-populists seem to be gaining even in countries, like Denmark and the USA, which have a large portion of mainstream media and politicians hostile to mass immigration.

The populist complaint against rightist politicians in the United States is that they claim to oppose illegal immigration, but that's actually just a scam to fool the rubes into voting for them. When push comes to shove, they quietly endorse the current (Democratic) administration's policies on the matter and make sure that efforts to block them always fail. Given that, the factors behind Trump's support aren't so different from the factors behind support for populists in Europe.