Top journals usually have distinguished professors from reputed institutions in the editorial panel. That's why we can rely (somewhat) on peer-reviewed articles in those journals instead of random blog posts or arXiv submissions.
> why it's important for some research on a rare degenerative illness to be known by people who are suffering said disease?
Well I can only imagine the pain and suffering such person and their loved ones would go through, but I guess it will give them some glimmer of hope and respite. Most people lack the expertise to understand and follow cutting edge research, so someone in a position to understand the research and simultaneously benefit from it personally should have access to the said research by all means.
That said, I think academic publishing is a criminal industry and Alexandra Elbakyan a modern day superhero.
The publishers' profit margins are one thing; anyone could start a competing publisher tomorrow and take a fraction of the profit. But people are not asking for reduced profit, they're asking for abolishment of paid publishing, which is like saying, let's abolish the entire system of scientific research.
Journals are gatekeepers to funding, prestige, academic disciplines, careers, advancement, etc., etc.