alt Hacker NewsI don't see why there has to be a "gap" at all. The country had half as many people in 1950, and it got along fine. The world is different now, but I don't see why it takes 300 million people to run the country today.
If the US gets back to 150 million people, it will look different, but I don't see why it has to be any worse than it is now. And I can think of a lot of reasons why it might be better.
Get rid of Medicare and Social Security, and the transition would be fine. The problem will be in transferring more and more wealth from the working and giving it to the non working, resulting in an everlasting loss of quality of life for the working (absent sufficient advances in automation).