Seems to be a universal thing, in a society that values capital as one of the highest achievements, it will co-opt anything that ends up trying to work against it and sell it back to you.
Medium and Substack aren't mediums, not like spoken word or watercolor or dance. They're platforms with admins who can be compelled to make changes with no regard to the consent of the users.
Things with that property do always fall to corruption. But we can build software that doesn't do that. It won't be worth investing in, because what investors are buying is the ability to compel that admin, but we can make it for ourselves of we want to.