If you wait in a line, it would be better for you to have less people in front of you. That does not devalue the life of everybody in front of you. Same story for demographics. There are advantages to having a shrinking population. That does not mean that it is the morally right call to commit mass murder or something like that.

The reality is that fertility rates on all continents, but Africa are below replacement today. That means many countries are going to have shrinking populations in the near future. Obviously that has consequences and at least I do not see a problem in discussing them.

Also no demographics are not going to destroy capitalism by themself. However less workers means workers gain power. If and how they use that is a different question and the fact of the matter is nobody knows. It is a bit like watching a weather forecast and complaining that it does not include a call to revolution.