The initial appeal of Medium and Svbtle, which was the other one I was thinking of, was that almost any time you saw an article from them it was usually high-quality. With their simplistic dark-on-light themes they looked visually distinct (especially Svbtle) from the popular blogging platforms at the time. There were no ads, no calls to action, no fucking modal "you need an account to keep reading" popups, etc. I seem to recall that Svbtle and Medium both began as invite-only, so the set of authors was highly curated.

Thus, the reading experience was fantastic.

As soon as they opened up to everyone, almost right away the quality dropped. All of a sudden Medium in particular was chock full of shallow "man page disguised as a blog post" posts and "tutorials" from people trying to build their own personal brands. It became a firehose of mediocrity.

Substack is currently experiencing the same cycle.

Ultimately I think, if you want to preserve the elite/luxury/exclusivity reputation, you need to impose artificial scarcity and resist the urge to "hyperscale" or whatever.