alt Hacker NewsRobots may fill the gap. Really. It seems silly now, but give it twenty years. The developed world may end up with a modest human population and a large robotic population. Asimov explored that idea in SF decades ago.
The humans may still think they're in charge. They won't be.
I 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.