What you describe would be some kind of a natural selection among lower level abstractions. Maybe they would indeed converge into a "few standard engines", maybe not. Either way, they wouldn't force your hands: you could always use whatever UI framework or network library you want and still distribute your app as a simple URI.

In contrast, "the modern web as it is now" gives you a single built-in layout engine and not even a chance to implement network libraries. And that's exactly my point: For a document viewer, that's fine. For an application distribution platform, not so much.