Then how does that explain the shift right though, many younger gen Zers are now more conservative than their more liberal millenial parents. Or how MUCH MORE liberal the millenial generation at large was compared to Gen X.The last 30 years saw a TON of changes in the social landscape because of that. Probably one of the biggest social shifts in attitudes in history in such a short period of time (look at how gay people were treated early in the 1990s compared to today, even by conservatives. Even they have largely come around to accept gay marriage).

Very often kids also tend to develop the opposite political spectrum from their parents, it depends on their relationship to them. If they like their parents you are more likely to see opinions align. If they don't like their parents you are more likely to see them develop an opposite stance.

The point is even should leftists stop having kids it wouldn't mean the end of leftism. It would make a comeback with a vengeance within a generation because now that for a short while no liberalism existed, naturally all of the world's faults would be blamed on the only ideology there is in that case… conservatism.

I.e. Atheism in the US is largely the result of spite towards a highly religious environment which is why the American brand of atheism is so much more obnoxious than the European brand, since religion in Europe is not as obnoxious either, so you don't have as much of a violent spite reaction.