> But it has had man-centuries of work to get it where it is now; a de novo solution will necessarily need similar levels of work to get to a similar point.

That's not true. There is a lot that went into X that is no longer in use, and doesn't need to be faithfully recreated; e.g. basically almost all rendering (font and otherwise) has moved from server to client. At least in the past (Latest sources I studied were of X10 ... showing my age), that was a significant part of X, with a lot of work going into it - and it does not need to be redone.