> The other bad pseudo-privacy idea is time-limited posts ("stories" or snapchat or whatever).

That one addresses a real threat-model - someone who was your friend and whom you trusted at the time you sent the post, but has since turned against you. That is quite common among teens, which is the main demographic these social networks are targeting. Rather than them having a permanent record of all your online interactions since you met including embarrassing photographs, it just becomes he-said-she-said like it has been for ages, unless they were actively subverting the program to archive content while you were still friends, which is rare.