By American standards I guess I'm pretty left-wing. Nothing that Sanders is proposing is something we don't already have in Europe.
Study after study shows that higher education correlates with lower reproduction. Higher quality of living in general equates to finding less and less value in having kids. It can be seen in Europe: no matter how much the governments try to promote breeding, birth rates continue to fall. Improving quality of life can be both a humanitarian aim in addition to serving the interest of population reduction.
Furthermore, I'm not a billionaire and I'm never going to be. Keeping income inequality to Nordic levels personally benefits me, even if I object to inequal redistribution based on breeder privilege. My healthcare costs are low and predictable, and I can't see how I'd be better off by exchanging them for instability, which won't even come with the guarantee that no breeder will get a cent from me. There are already a plethora of breeder-benefit programs in the US (like WIC), so it isn't as if it's a choice between equality and inequality for the CF - the inequality is a given so the real choice is between maximizing my own benefit and cutting off my nose to spite my face. I don't think the CF have anything to lose by joining in the metaphorical eating of the billionaires, as unpalatable of a sauce as the "children and grandchildren" rhetoric may be as an accompaniment.
That's of course without taking into consideration human rights. I would never vote for someone who was obviously racist, sexist, opposed female bodily autonomy, or who supports sexual assault. I don't care if they promise to give me a grain silo filled with gold, and the alternative choice is going to leave me broke. I find it so extremely unethical to deny a woman an abortion that I could not support someone with that view.