I have a noob idea and I want to understand where I'm going wrong :)
Say nodes agreed on a block revision limit, say, 6. This would mean that if they see an incoming chain that is longer than the previous longest chain they have seen, they would reject it if it involved changes in blocks further back than 6 blocks ago on their previous longest chain.
If vendors then required 6 confirmations before delivering purchased product, they could be sure that the majority-agreed chain will not be revised to double spend the BTC used to purchase that product. This would protect against a 51% attack.
What am I missing?