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Substack continues growing, it looks sustainable and profitable, and unlike BlueSky isn't attracting only one part of the political spectrum. I don't see Substack doing much to raise the sanity waterline, but it might be a worthy target for some rationalist lobbying to improve their architecture.
Substack has a great business model, but from technical perspective it sucks. Good enough to post articles and send them to paying subscribers -- which is where the money comes from -- but reading a ACX-size comment section is painful. Also, as an author, if I want anything non-trivial, like making a word simultaneously italic and a hyperlink, or to choose a smaller font size so that I can have code blocks with 80 characters per line, no chance.
Unfortunately, the scale and network effects mean that "make another Substack, but one that doesn't suck technically" probably isn't a good business idea. Or rather, would require a very large investment.