"the company looked at the history of social media over the past decade and didn’t like what it saw… existing companies that are only model motivated by profit and just insane user growth, and are willing to tolerate and amplify really toxic content because it looks like engagement… "
There's a very large number of DOM and browser APIs now though… Even with basic JS without libraries or frameworks, you can still build fancy 2D and 3D graphics (WebGL), interact with USB devices, allow input via game controllers, stream H264 video, implement custom caching, use push notifications, and a bunch of other things. The web browser has to implement all of that complexity. They're all useful in different scenarios.
That's a good point, I guess I haven't been too aware of all that stuff.