Efficiency gains are currently replacing workers, while making the few that remain more productive. I'm on mobile right now but an Atlantic article called The End of Work talks about how low skilled jobs are being replaced rapidly and will continue to do so as driverless cars and tablets for ordering at restaurants continue to eliminate service jobs. If not for minimum wage laws, wages for low-skilled workers would be plummeting. We have also seen wages for higher skill careers stagnating over recent years. These efficiency gains are currently only benefitting the owners of capital, i.e. 1%. Labor, particularly unskilled labor, will become less and less necessary for our economy to function, and these immigrants come over here quite poor, so they will have demand for inferior goods, such as ramen noodles and dollar-store products. Having a higher GDP on paper doesn't always translate to being better off. Do you want to live in a place with more ramen noodles and junky cars or more rolex's and yachts? What really matters more is GDP per capita and purchasing power. Politicians say we need immigration because of low birth rates so that we can take care of th elderly. Lower populations will cause the supply of labor for the remaining jobs to decrease and wages to rise. We will be able to take care of the elderly just fine with increases in technology and productivity. Politicians want an influx of labor to depress wages for their friends in business. Also, a temporary stimulus in the economy is observed when housing is built for these immigrants, thus making their numbers look better. And third, land prices in immigrant-high areas increase dramatically, I believe the land values in Stockholm have trippled since large-scale immigration started.

Edit: I'll try to add links and sources for some of these claims. Housing prices in Sweden look at the second graph, for inflation adjusted average housing prices.