I think you're confusing the implementation with the idea. XUL wasn't the right path forward for bringing web technologies into the desktop as we can see with the failure of XULRunner but a ton of apps are written today with Electron and HTML5. E4X was an interesting idea bringing XML into Javascript as native objects, but the ultimate implementation lives on as JSX with React.
As someone who had to maintain E4X within SpiderMonkey: sadly, that's another great example of the wrong technological path. JavaScript is too dynamic, it gives you too much rope to hang yourself, and E4X was apparently not designed with that in mind. It was so nice when we removed E4X and didn't have to worry about `x` being an object during a lookup `obj[x]`.
To give an example of the flavor of issue: when building an array (even from a literal), you couldn't push a series of values to the stack and then construct an array out of them, because with E4X between elements 2 and 3 you might end up invoking arbitrary code with access to the array so that it establishes a setter on element 4 and adds a `valueOf` hook to what you're passing as element 5, but oh wait that's the same object you already inserted as element 0, and anyway the array might be frozen now and... (I'm making this up; I know the issues were similar to this but I probably don't have the details quite right.)
E4X was a rich, rich source of security bugs. I'm sure a simplified form of it is possible and would be quite cool, but that niche is already filled now with JSX for better or worse.
