AFK timers are a holdover from primative game design, back when world servers used a single process and could only have a limited number of players in them. MOST modern MMO's don't need AFK logouts, because every zone is instanced anyways, and an idle player doesn't use much in the way of resources.

Apparently, FFXIV's design is NOT very modern. Even EQ2 (a 9 year old game) would simply create new instances of the main city zones if they got over-populated.