Conservativism has to reproduce at higher than replacement rate to persist because it constantly loses people to progressivism, net. Progressivism is sustainable at lower-than-replacement-rate fertility because it generally gains converts, net. Memes eat genes
tribal recruitment in times of intense social division is one thing. I have sincere doubts that what you're calling "conservatism" (which has been meme converting rather furiously, not sure why you haven't noticed) and "progressivism" are coherent as ideologies at this point.