Starting a war only kills certain demographics, namely young poor people, as well as anyone who lives in a place that gets bombed.

Police handle other demographics.

That worked well enough back in ye olden days, but you don't see it much in the modern world. Winning a war isn't just about having enough people on the front line. It's about what kinds and what quantities of weapons and equipment you can get to people that matters. A tiny country with a small population and an advanced industrial economy will wipe the floor with a country with an army of peasant conscripts 10x as large. Keeping the people in uniform equipped and fed is even more important than actually finding people to put into a uniform.

This matters because every person you send to the front line is another that can't be on the home front working at a munitions plant, or working on a farm, or driving a train to transport war supplies, etc.

Unless your economy already has structural mass unemployment, there's no such thing as a free lunch. Every draftee you throw away into the meat grinder is one more that can't be supporting the war from the home front.