So then, where is the barrier? Is this an argument based in Moralism, or Materialism? The Proletariat can and must do whatever it can to improve its own conditions, and in the absence of a state that provides for it, this must be done via the Market.

I agree that Workers, especially Marxists, should try to reduce their harmful footprints as much as possible. Boycott union busters and supporters of genocide. However, when it comes to retirement savings, there exist no ethical ways to move forward, except to live entirely off the grid and refuse to use banking systems.

Unfortunately, this can lead to reactionary behavior, a turning back of the clock to an earlier stage of Capitalism. That's why homesteading has a huge fascism problem.

I guess what I am trying to say is that I don't think it's productive to say investing is antithetical to leftism, considering the context. That would be like saying purchasing video games is antithetical to leftism, as they are for personal enjoyment yet support Capitalists. I try to think in terms of Dialectics and what materially benefits the proletariat, and ultimately if it becomes a matter of purity testing I believe we lose sight of the goals of Communism.

I dunno, this became a bit of a ramble. I'll end this with a fun quote:

“The moment anyone started to talk to Marx about morality, he would roar with laughter.”