Apps that let you manipulate artifacts are often local. MS Office, Photoshop, Blender, CAD tools.
But it turns out that actually, humans rarely work or live alone, and you can't facilitate communication through a local-first app.
Yet somehow people managed to do that before everything went to the cloud.
Surely an email client like Thunderbird is local first? Even the email apps on mobile devices work offline and send and receive when the get a connection.