That's today, in 50 years I will say the demographics are going to swing far away from the left. Whoever remains of the left will be medicated and kept in isolation tanks with VR goggles while they imagine that they are dolphin-sexuals, elves or king of the world in their own fantasy Meta universe where they ride in on a unicorn and blow fairy dust on bad guys. They will have human to peanut butter sandwich operations in their universe along with PB&J being new pronouns to describe them.
Demographically though the far left is not having children, but the far right is. Lots of people start off on the far left in life because it's the easiest place to start from. Everything else takes actual time to digest and understand, critical thinking and the ability to ask difficult questions about one's own personal motives or the realization of 'how do I wish the world to be' versus 'how the world really is'. Some people can never get past that, but by not breeding they are going to slowly disappear from the social consciousness and the population by their own hand. In several decades they will be angry, pissed off, middle aged, blue haired incels who deeply regret their sex change surgery and the resulting sterilization while they watch the conservative family happily playing with their kids at the park. Something they can never have. They will be even angrier when they realize that they will die alone with no one to be with them. It will anger them that they can never have that because they lived in a blue bubble where everyone golf clapped everyone else's madness as a leftie purity acceptance test and they themselves were golf clapped onto the operating room table and no one else would dare to whisper in their ears "are you sure you want this?". But they will remain largely ignored by everyone else, in much the same way that the 70 year old blue haired worn out hippie chick in aisle 5 of Whole Foods stomps her feet and stammers over the wilting kale is ignored today.