Developed countries do not have to admit mass quantities of foreigners. They are choosing to do so, in part because they do not have sufficient workers/taxpayers among the ethnic-native population.
Cause and effect, chain of events. One can project as much as one likes, but most countries have self-generated their own set of problems through the actions of their own citizens.
The US brings in 1.1 million LEGAL PERMANENT IMMIGRANTS a year, the highest number in the world by far. Since 1990 over 35 million legal immigrants have entered this country, which is equivalent to the current population of Canada. Do we have a shortage of workers.
US labor participation rates (1980-2018) ages 16-65:
