I don't believe the Extinction exists (in-universe). Every "Extinction" statement is actually another entity (mostly Stranger or Spiral) that just happened to have an apocalypse-themed incident. The concrete jungle one is Stranger, the theme park is Spiral, the apartment door is Spiral, etc. I can explain every last one of these and why it makes more sense as one of the 14 and not a new Fear. The "Extinction domain" in s5 is Desolation (destruction of home, routine, and familiar things; no recourse or ability to cope; ashes of your life and world). Jon was able to pull off the Watcher's Crown 2/Magnus Archives ritual without an Extinction mark, so it's not separate enough to need to be called separately like the 14 are. At this point I don't even really care what the authorial intention was about if the Extinction is real because the text supports my theory