For me, the thing that kills the theory for me is the idea that,
in the middle of western Europe being about as politically stable and integrated as modern Afghanistan, all the warlords pillaging the country to kill each other were able to not only unanimously agree on "let's change the year to a nice round number", but were also able to coordinate this without leaving
any historical records of it.
Here's the thing about conspiracies: they have a shelf life. The amount of time you can keep a conspiracy secret is bounded by the number of conspirators, how well you can get them to shut up, and how long it has been since the conspiracy occurred. You can take a secret only you know to your grave, but work in a group of ten and someone will confess before their death. Bring in hundreds or thousands and the conspiracy will be blown open accidentally before you can even execute on it.
Now think of how many people had to cooperate to change something as simple and fundamental as the passage of recorded history. Hell, forget the fact that these were all angry, violent warlords; just think of how many scribes, monks, and bureaucrats would all have to remember to write 1000 instead of 736 whenever they wrote a date. It only takes one slip-up to reveal the fraud to archaeologists in the far future, even if they all agreed that they wanted to live in the nice round-number millennium. And nobody would be trying to protect against that because nobody would be thinking, "how do we hide this from 21st century archaeologists trying to discover our time crimes?"
Here's the thing about conspiracies: they have a shelf life. The amount of time you can keep a conspiracy secret is bounded by the number of conspirators, how well you can get them to shut up, and how long it has been since the conspiracy occurred. You can take a secret only you know to your grave, but work in a group of ten and someone will confess before their death. Bring in hundreds or thousands and the conspiracy will be blown open accidentally before you can even execute on it.
Now think of how many people had to cooperate to change something as simple and fundamental as the passage of recorded history. Hell, forget the fact that these were all angry, violent warlords; just think of how many scribes, monks, and bureaucrats would all have to remember to write 1000 instead of 736 whenever they wrote a date. It only takes one slip-up to reveal the fraud to archaeologists in the far future, even if they all agreed that they wanted to live in the nice round-number millennium. And nobody would be trying to protect against that because nobody would be thinking, "how do we hide this from 21st century archaeologists trying to discover our time crimes?"