They can’t change the birthrate decline fast enough to make that a go for a good long while. Look at the enlistment numbers, they’re at a low. It would be a draft. And our current society isn’t draft material. Most of us wouldn’t work well as soldiers, and those that would probably wouldn’t comply. In addition, going to prison would be free housing, meals, and medical. The food would be crappy, but even so, it may serve as motivation to NOT comply.