Here are my 2 cents (long post, sorry):

Current status:
 - Long form posts today on Nostr are difficult to discover because most clients don't support it and instead you need to go to a Nostr enabled web site like Habla.news to see who has posted something lately. This is very suboptimal

 - Outside of Nostr, subscribers to newsletters receive them via email. They don't need a new client. They discover the new issue of the newsletter as part of their existing habit of checking their email

 - The key here is that it's an existing habit on an existing client. No need to do anything new to discover new issues of the newsletter you subscribed yourself to.

Possible solutions:
 - When a publisher posts a new issue on their newsletter, Habla.news could simultaneously create a kind 1 event with the summary and the URL to the issue. This will allow the publisher's followers to discover the new issue through their existing habit and client, even if the client doesn't support long form notes

 - DM to all followers of the publisher. Could be considered obtrusive but let's face it, you're following that person for a reason. 

- Using a public list for user subscriptions like nostr:npub107jk7htfv243u0x5ynn43scq9wrxtaasmrwwa8lfu2ydwag6cx2quqncxg suggests above will work. But, doesn't this defeat the purpose of Nostr? I have always thought of Nostr as a way to follow a person and all the content this person produces. Would the public list require me to take a second step of subscribing to the publisher on top of already following him/her?