It's the same problem as streaming services and a-la-carte programming. Maybe a side effect of subsidizing writers instead of subscriptions; it'd be easier to pull a "subscribe to 2 writers and get a 3rd subscription on us"-style deal and let the readers figure out who gets paid that Substack-funded bonus if they were spending more money on reader growth than popular writers, but it'd be less likely to result in the kinds of content they have now.

Most writers have free tiers? You can tell which posts you're missing by any given writer when you're on their free tier because it'll show the post as locked. In most cases, it's not stuff I'm interested in. But maybe I'd pay for The Diff someday...