> building a truly local-first anything is significantly more complicated than the alternative of "always online" for most domains

It is more complicated. It works best when the user has a distinct data bucket that is exclusively theirs. A note taking app. Personal finance. But they may have multiple devices so syncing is desirable.

This use case is reasonably common and turns out to be relatively simple to do.

If you have shared data between users. If you have a significant amount of data that just requires being online to interact with the system. Then it has pretty minimal benefits over just centralizing the data and it should be avoided to minimize complexity.