No one gets all worried over every calendar found all ending on one magical day, December 31. Horror of horrors, what did the calendar profits know to have all calendars end on exactly that same day, does it mark the end of the Universe ? No, it's just time to go buy another calendar.
It's the same for the Mayan rock, who ever owned one just needed to go out and buy another. The problem for us is there are no Mayan calendar rock carvers about to provide a new one. ( a funny side note - that old worn rock can still be used to keep track of the next cycle, that's if no one already X'ed out all the days )