I'm thinking about this because I want to build a Nostr app for my selfish use case.

Login with nostr is optional. But it helps sync cross platform.

If you're not logging in there's no sync. It's that simple. Like obsidian. You don't need to use sync, but if you want it, you either figure it out or pay for the service.

With Nostr, I'm not as familiar, but the alternative could be like self hosting on your relay is free, but special Nevents to get your app to sync can be reserved / hidden behind paid relay, hosted by the developer of the Nostr App.

On top of that, here's an additional design feature to bolster adoption:

Downloading from zap store is free, downloading from Playstore / app store is paid.

I'm not a developer but I think what I described is possible? 🕵️‍♂️

Will be exploring this as I'm testing out vibe coding a nostr app