Sure, it's simple, but a stated goal is power to weight ratio. Transclusion definitely fits under that. Authoring and organizing information with transclusion is so much easier than without, because it avoids much of the need for special tooling to assemble a final document from fragments that might be reused. It's too unwieldy for very large documents though, so it's perfect for authoring small to medium sized documents, which fits right in with Gemini's goals.

There are many ways to handle it client-side:

1. Behaviour like iframes, although I don't like the tracking implications.

2. Origin server fetch, as I mentioned.

3. Another possibility is to render it as a button which the client must trigger to load the document.