X and its evolution is something that applies to all of the degrees of reality on the ‘virtuality continuum’ of XR. In order to be a game-changer, X needs to be something that, at least in part, currently holds the market adoption and content quality back.
In this article Aki Järvinen rewords the question.
https://virtualrealitypop.com/what-s-the-x-in-xr-715bb274a0f3