I don’t think I 100% understand what you mean by population decline? Do you mean planned parenthood/contraception? In other words: what are some of the strategies you are in favor of to make this decline happen?
birthrates in the west are trending down. the US hit a new low recently of 1.8 births per woman, well below the replacement rate. the US currency has a slightly positive (0.5%) population growth, but this is only due to immigration. if we merely do nothing (and don't adjust immigration quotas), we will have negative population growth in the near future.