For many years, the X Window System has been a core component of every Linux distribution, as well as the BSD UNIX variants (with the known exception of Mac OS-X), next to the kernel itself. It was the X Window System that provides the basic graphical interface on which our desktops were built.
When I first started using Linux back in December 1998, XFree86 was available, and this was X Version 11, Release 6.3. X Window System was first developed back in 1984 at the Massachusetts Institute of Technology as a project for the development of the graphical desktop that can work with any hardware that was available.
Hardware at that time ranged from terminals with graphics modes such as those manufactured by Tektronix, to Hewlett-Packard workstations equipped with tablets and plotters (that used pens instead of ink cartridges), to the original IBM-PC, XT and AT.
The X Window System was commonly used with three button mice, which were essentially bricks with wires attached and connected to a standard serial port or a proprietary hardware port (USB was not around then).
The typical use of the X Window System was to have multiple terminals open using one keyboard and one mouse. A few gadgets were available, such as a mail notifier, eyes that moved with the mouse pointer, and a clock. Of course, these were primitive in look compared to what we have today.
However, the X Window System was plagued with performance problems, as well as numerous bugs and security concerns. This is one reason why it took more than eleven versions of the software, and six releases of the eleventh version, to get it right for adoption to our desktops…and it took fourteen years of development to get it to where we can actually use it on a day to day basis.
The current version of the X Window System released from X.org is X Window System Version 11, Release 7.3, and everything works as expected.
Now, after twenty seven years of development, there is a new project in the works to eventually replace the aging X Window System.
Currently, Wayland is implemented as improvements to the way X does things as far as interaction between clients and the X server, and can coexist with the current X Window System.
So far, only Ubuntu and Fedora have plans to adopt the new Wayland system.
Remember that it took fourteen years for X11 to get it right for production use. If Wayland is to replace X11, the API functions would have to be compatible to the point that little recompilation is necessary to get the desktops working, and the Wayland project itself would have to have all components working properly to get up to speed with the rest of Linux development.
Imagine having to rebuild LibreOffice, GIMP, Firefox, and Thunderbird to work with Wayland, not to mention the amount of work required for Linux distributions (other than Fedora and Ubuntu) to adopt the new Wayland system.
This is important because the X Window System is the core component of Linux (and other UNIX) on which every graphical desktop (KDE, GNOME, Enlightenment, XFCE, LXDE, WindowMaker, etc), every hardware driver with a graphical interface (SANE, XSANE, CUPS, HPLIP, etc.), every major application (including Firefox, LibreOffice, AbiWord, GIMP, etc) was built. In short, if X11 does not work, most everything else will not work.