Why not just allow people the option of privatizing their retirement contributions and stopping this eternal cycle that only would have worked with a bottom heavy population pyramid.
Getting closer to stable population seems fine with me, for significant decline I question whether the push towards saving and investments are a real solution. Sharply declining workforce would render a lot of the earlier amassed capital worthless and devalue the remainder as all capital would be in surplus for the needs of available labor. Our current paradox with automation would really have to come to an end.