A Look Back At The Desktop & X.Org/Wayland/Mir Milestones Of Ubuntu
February 2013: It was in February 2013 when we heard that Ubuntu was working on its own X/Wayland alternative.
March 2013: Mir was announced as Ubuntu's X.Org/Wayland replacement. At the time they felt Mir would be ready for Ubuntu Phone OS by October 2013 and ready for the desktop and more in 2014. Mark was quick to call Mir "a fantastic piece of engineering." It was also when they first started talking too about Unity Next, what would become Unity 8. Ubuntu developers were quick to begin showing Mir / Unity Next demos.
April 2013: Unity 7.0 is the newest version of the Ubuntu desktop for Ubuntu 13.04.
2013: Ubuntu Touch becomes the project focused on getting Ubuntu on tablets and phones. Early trials in Benchmarking Ubuntu Linux On The Google Nexus 10 and Benchmarking Ubuntu Linux On The Google Nexus 7.
Their goal was to have Unity 8 and Mir as experimental in Ubuntu 13.10 while a polished Unity 7 and Compiz would be the default. Their goal then shifted to shipping Mir by default in Ubuntu 13.10 with an X compatibility layer. It was also around this time that Mark Shuttleworth himself called Mir a performance win.
October 2013: Their goals weren't met and (X)Mir wasn't ready to be the default in Ubuntu 13.10.