This is exactly how I use Syncthing, and as the author says, it sure would be nice if more things were just files. Really, most things are stored locally as files, but not always in a way that plays nice with syncing. Like, I can sync my Firefox profile between machines (it’s all in one folder), but I found it prone to conflicts, with little to resolve those conflicts.

In a similar vein, local-first apps built with Conflict-free replicated data types (CRDTs) can be another way to avoid server dependency. I haven’t seen any significant apps built this way yet (just occasional blog posts about it). I imagine the CRDT approach would work better for individual apps, since conflict resolution can be written in a way that works best for a given app, but I also imagine that such apps would not play nicely with a generic sync solution like Syncthing.