Any system that is not being manufactured anymore is technically dead.
However many old systems still are in use by enthusiasts like the people you find here and other websites.
I myself have a number of Amiga systems here at home with me , but my favorite is still my old Apple IIe systems - mainly because I grew up with them so have fonder memories of the Apple rather than the Amiga. Though the Amiga can do so much more than the Apple II could.
Many old systems etc are still valid for many people , I still know people using SNES and Megadrive consoles , heck I know someone with an Atari VCS.
I have all the modern consoles but I still can't get past games like Super Frog or Great Giana Sisters on the Amiga , or Odyssey or the original Wizardry on my Apple IIs. Even more validly , these systems are still working ( generally ) sometimes after 20+ years - I doubt we'll see a PS3 or XBox360 last that long.
Also to add to your list of "dead" pcs you can list the Hitachi Peach - not sure what name it was sold by in other countries , but it came out about a year after the Apple II. I remember being very tempted as the graphics were better. I have a feeling it may have got lost very quickly.
Also there was the Tandy TRS-80 - this was very popular in it's day , even producing clones called the System-80 here in Australia. Always remember the game Time Trek on this as being my fav. If you put an am radio near the pc , you could get "sound effects" and get a noise when the Klingons
were getting ready to fire etc.
Got to love retro gaming!