Why take a cut from the publisher? Why not instead open up access to the publishers via a marketplace on your site so people/companies can sponsor the mailing lists? You can take a cut from the sponsor for having given the opportunity to introduce them to publisher(s) that cater to their market. It's a win-win-win for the potential sponsors(advertisers), the publishers as well as Substack. As a sponsor, I would have immediate market access through these publishers who cater to my niche. As a publisher, I would have outsourced the task of attracting sponsors and managing sponsor timelines to Substack so I can focus more on creating content and expanding my mailing list. As Substack, you wouldn't have to annoy your publishers by taking a cut and instead carry over your fee to be paid by sponsors/advertisers. A bonus would be allowing your publishers to carry over their existing sponsors to use your system which would possibly attract a huge populace of publishers who are currently manually micro-managing their existing sponsors and timelines instead of focusing on creating content.