Substack is winning because of business model innovation, and also because of market timing. Their technology isn't really an advantage compared to legacy CMS platforms - how they are employing that technology is the difference.

Medium went for a bottom-up approach, trying to monetize the content of the crowd. Substack is going for a top-down approach, grabbing writers with an existing large following and paying them top rates, which are justified by the conversion rates for their well-known creators.

Normal people might not have heard of these companies, but niche audiences certainly follow some of the specific authors/columnists/influencers and are following them onto substack with paid subscriptions.

Is that a moat, or will they jump to the next platform? A few have been poached by the NYT and other venues, so maybe the moat isn't that big.

Ask any writer you know about substack, they have mind share that the competition will struggle to catch. It's a moving target remember, first you have to launch, then you have to get a load of a-list writers on your platform, then you have to figure what new thing is going to get you users. This will take you at least 6 months maybe longer. By then Substack should be at least 6 months ahead of you maybe more.