To be fair, implementing something like the Spotify client does require a web app, not "just" a web site.

But even document rendering with light scripting is not trivial so yeah, the required browser is the bottleneck.

I always wonder (layman question): couldn't native Electron apps (and similar technologies) save a great deal of RAM by using the same sandboxing model for apps that browsers use for tabs, instead of fully-fledged instances?

Was that an idea that Tauri also tries to implement, or am I remembering this wrongly?

It'd be great if Electron itself became a core part of the OS. Tauri is closer to this on MacOS because it tries to use WebKit but not enough.