From:
https://wayland.freedesktop.org/faq.html#heading_toc_j_4> ## Is wayland replacing the X server?
> Mostly, yes. User sessions are able to run under Wayland today, via a number of compositors: Weston itself as well as Enlightenment, GNOME Shell, KDE, and a number of others under development. With most toolkits having Wayland ports, as well as frameworks such as GStreamer and SDL, it's perfectly possible to run a purely native Wayland session as your desktop.
> That being said, there are some clients which rely on X11, and always will be. To that end, XWayland provides a plugin for Wayland compositors, running a real X server. This gives legacy clients a real and compliant X11 platform to run on, displayed side by side with native Wayland clients in your Wayland session.
> ## Why not extend the X server?
> Because for the first time we have a realistic chance of not having to do that. It's entirely possible to incorporate the buffer exchange and update models that Wayland is built on into X. However, we have an option here of pushing X out of the hotpath between clients and the hardware and making it a compatibility option. What's different now is that a lot of infrastructure has moved from the X server into the kernel (memory management, command scheduling, mode setting) or libraries (cairo, pixman, freetype, fontconfig, pango, etc) and there is very little left that has to happen in a central server process.
> ## What is wrong with X?
> The problem with X is that... it's X. When you're an X server there's a tremendous amount of functionality that you must support to claim to speak the X protocol, yet nobody will ever use this. For example, core fonts; this is the original font model that was how your got text on the screen for the many first years of X11. This includes code tables, glyph rasterization and caching, XLFDs (seriously, XLFDs!). Also, the entire core rendering API that lets you draw stippled lines, polygons, wide arcs and many more state-of-the-1980s style graphics primitives. For many things we've been able to keep the X.org server modern by adding extensions such as XRandR, XRender and COMPOSITE and to some extent phase out less useful extensions. But we can't ever get rid of the core rendering API and much other complexity that is rarely used in a modern desktop. With Wayland we can move the X server and all its legacy technology to an optional code path.