If you want a peek at the future, try looking at Japan. It's a sobering exercise. Here's how economist Timothy Taylor, managing editor of the Journal of Economic Perspectives, describes the country's outlook:
"[Japan] is facing a situation of a declining population and workforce, and the share of the population that is elderly is on the rise. [This is] driving up government spending on pensions and health care, and together with attempts to stimulate its economy through government spending (much of it on infrastructure), Japan has run up an enormous government debt."
To put it bluntly (as I have argued before): Japan is slowly going out of business; its population is shrinking and it resists immigration. This cannot continue indefinitely.