So you want an aggregate newsletter with the top posts of the last X days in your email and then a link to substack? What part of that experience isn't possible on the web?
Yes, I'd like an aggregate email notification, sent no more than once a week, containing only the titles of recent posts of all the newsletters I subscribe to that will allow me to go to one place, like an app, where I can read all these articles at once rather than piecemeal. Prior to the app, the only way to get notified of a new post was to get an email from the newsletter, which as I mentioned can start to become pretty tedious because of the constant stream of emails. Alternatively, one could bookmark all the newsletters one subscribes to on a web browser and keep checking them, which is also quite tedious. A reader app is a better solution.