Comment by nvm0n2

People who are serious about this topic should start by studying past examples. Once upon a time all software was local first, and there's plenty to study there about what changed and why.

The masters of this game are Apple. Bar none. All their software is local first and asynchronously syncs to iCloud (optionally!) using CloudKit, which is a relatively consistent replicated database. This design is hard to master but worth starting with. They also support P2P that works (Bonjour and friends). Apple can do this because they're firmly rooted in the software development milieu of the 80s and 90s. They never got the memo about the web taking over and revel in writing high quality desktop apps. Users love them too. Developers like to pretend this ecosystem doesn't really exist outside of mobile, but it very much does.

There are a few things that make this hard for other kinds of developers to do well.

One is that it requires apps to have file formats, and most devs don't learn these days how to design them. They know SQL but not how to wrangle complicated byte arrays. And our infrastructure kinda sucks at working with files. You can't POST a directory over HTTP, for example. SQLite helps a lot here, but you still have to publicly document your schemas if you want the user to truly have a chance at "owning" their data. The cloud then just holds a backup of these files.

The desire for strong collaboration and sharing features is another problem. Apple isn't particularly big on those. However, a lot of software barely needs it! In most cases a way to publish or share documents with a hyperlink is good enough, especially if others can comment on it. A lot of users don't actually want to allow unrestricted real time edits to their work as it undermines ownership, hence why Google Docs has now moved to a more Word-like workflow where edits are help in suspension until explicitly accepted.

The biggest problem is that the only form of sharing that actually works well is an HTTP link, due mostly to OS makers catastrophically dropping the ball on anything else. And that pushes people towards the web. Apple solve this with light web renderers for their file formats that let people view read only copies, more or less. Often this is sufficient.

The Apple approach + an openly installable backend is a reasonable way to do local-first software. It doesn't imply the software has to be free or open source. It can still be proprietary, you just need to sell the backend as software as well as the frontend.