They should leave them up. There really is no reason to take down the pageantry tome stones after the event concluded, it was largely arbitrary and they kept prior exchanges up after the event for the last three times I think. I still got 130 of them and didn't even realize the event was honestly just going to end like that.
And even if you did know that it was going to end, would you honestly agree to have them taken down? Why leave the other ones up, and just take down this one? It's not about whether someone remembered to turn them in or not, it is about the whole goal of having these tomestones up in the first place. If someone went on vacation or got busy in life, then missed the cutoff date, and they had a huge store of tomestones stacked in their inventory they just got burned by the entire system.
And FYI just about every single game in the universe lets you dump the items into something from events after the event ends, or makes the items you get from the event directly usable in some way. WoW had candies you could use during the valentines day events and while that one had chocolate turn ins that didn't do anything, I recall the reward was mostly just getting more expendable items. The pageantry tomestones are like a motherload of time consuming items you can normally get in game at great costs in time. I'd never take the darn thing off for at least two months after the event ended. Maybe give people a week or something afterwards minimum but just leave the turn-in moogle there.
Ah before I forget, if someone uses quick launcher, they probably didn't see anything about the event end being the point where tomestones are now worthless. I barely even come to the lodestone as is, I just came over here because I logged in and noticed the event ended, went to do my usual thing of turning in the tomestones, and they are gone. No warning or anything. Maybe it was in the purple text or something? I don't remember seeing it when logging in and if I did it got buried immediately in the chat log. I mean do we need another Garo train?