Sounds like that's a symptom though to me. Once they fall out of the mainstream, they get booted out and sent indieland.

I suspect it's a little of column A, a little of column B. :) While a theoretically great thing about Substack is that you have no editor, a serious pitfall with Substack is that you have no editor -- and, depending on publication, no research staff or fact-checking department. So there's no one to push back even with relatively gentle comments like "I see what you're going for here, but you need to support [thing] better" or "your snark here is magnificent but it's undercutting your larger point."