The problem is that's is very difficult to freeze population to a constant level (see China). You might be able to freeze the head count but run the risk of severly skewing your age pyramid, which can lead to massive problems a generation later. Moreover, birth control isn't popular in the free world, you'd be limiting an essential human freedom (and the purpose of life).
The danger is declining population.
You don't actually want declining population:
1) Most pension schemes rely on at least constant population. Smarter pension schemes rely on economic growth (which is possible with slightly declining population), but not all countries have them implemented.
2) Declining population can also trigger massive problems with economy: You'll have to divest in a controlled and smart way. Example: real estate values are likely to drop if head count goes down. See former East German towns: some of them have become almost ghost towns, many with only retirees living there. This triggers business closings, which in turn makes young people move away. A self enforcing negative trend.
More population is no problem. There's lots of space on earth. If it becomes too crowded people will move to Mars or space. In fact, that could become a driving force, eventually.