Why don't they have more kids????
You know, encourage the fertile women folk to put some love in the oven.
The sexual revolution wasn't only about bed hopping. It was also about attitudes about traditional sex roles for men and women. Japan has moved right along with more women with careers, delaying or putting off marriage, delaying or putting off having kids. Add it all up and it results in population declining.
Whether that's good or bad is a moral/value judgement, and if we're good politically correct drones, we won't question whether declining populations in developed countries are good or bad.