X11 isn’t going anywhere, there are plenty of implementations some free, some opensource and some commercial.
However, Xorg (an implementation of X11) is eventually going to be replaced under Linux by Wayland on most distributions and by Mir on Ubuntu, but will still be used for BSD and other Unix and Unix Like Operating Systems. Not to worry though, KDE, Gnome and The Enlightenment projects have already began work on Wayland, and canonical obviously has already started working on Unity Next with MIR support.
Nope, not at all. It’s just the display server, for the average user there will be nothing to worry about. The Desktop environment and window manager developers are the ones that need to do the work, which as I stated before has already began.
Wayland utilizes a compositor that off loads graphics to the GPU, this leaves the CPU to do what it was intended to, so in that sense, yes it would use little to no system resources for graphics rendering. There are other advantages to Wayland over Xorg but I haven’t got the time to go into them all.