As the functional programming folks figured out & are now telling everybody, mutable content makes everything harder.
1) if content is mutable, then links rot
2) if even blind one-directional jump-links rot, transclusion is even worse. One day you're importing an important diagram & the next somebody has replaced it with goatse.
3) and you can't trivially replicate or cache it
In other words: forget embedded markup, and instead do formatting & linking as an operation that works on pointers.
This is Xanadu 101 from 1965.
Actual access to documentation on Xanadu concepts (in any kind of coherent way, with any kind of context) is hard! I understand. I spent years trying to figure this stuff out before joining the project.
Read it. If you have questions, ask me & I'll clarify. You don't have an excuse for not understanding core XU anymore.