No, I'm pretty sure the Mayans just got tired of counting, would you sit there day by day carving 2012 years into rock? Yeh, you'd get bored of it.
Thats what the guy who make the calender had to do.
Mayans, Mayans, Mayans... Too bad they never heard of "shifts"... Eh, who knows why they stopped counting, maybe what you said, maybe some shaman high on ancient drug tought that theyre god tell them to stop, or something. Anyway, we will see what will happen.
When 21st comes i will be sitting home, on my couch with a nice can of beer and a full bowl of chips. Ey, well see!