I think focusing on birth rates puts attention on the wrong thing when it comes to talking about Japan or other post-industrialized countries. Even the replacement birth rate (i.e. keeping the current population) is at best just-barely sustainable from an environmental and quality of life perspective. Having too many retirees is indeed an issue, so I think the solution is to have fewer retirees. Realistically, a good amount of people could work well past the typical pension age without much of a sweat. They may not want to, but the cold fiscal reality is what it is.