Not much of an assumption. Many local-first solutions work from a big wad of JS. I've seen use of WASM to include things like SQLite. It's not pretty.

My point is that if you're in danger of losing your internet connection, you're not going to be able to reliably get that initial JS bundle anyway.

Yes, but "local-first" and "Big JS" aren't related. Sure you can build any monstrosity and make it local-first. And my local-first software can be a Tauri app, installed-once, or any kind of local software for that matter.