Point is that you have to be incredibly careful about allowing new complexity. Now we have reached the situation that implementing a new browser has become possible only for very large companies. This would not have been necessary if the web was built using a more layered approach.
That's too reductionist. There's value in having a media-player-like app of limited complexity and purpose, with a wealth of server-side platforms to deliver content.
Have you ever looked at your genome? It’s jammed full of old stuff tucked away in case it turns out to be useful someday.