Of course you can drain accounts with 50% hash power or greater. And the number isn't 51%, it is actually 50%. If you have 50% of the hash power, you will be ah

Of course you can drain accounts with 50% hash power or greater. And the number isn't 51%, it is actually 50%. If you have 50% of the hash power, you will be ahead exactly half the time. However, you will selfish mine and this creates an asymmetry. When you are ahead, other miners will mine off your chain, making your chain the longest. You will never mine off other published blocks, no matter how far behind you are. And no matter how far behind you are, you will eventually be greater than even at some point. Because you can start mining at any point in the past, you can erase all transactions except the genesis/first transaction. The bitcoin developers realize this so they essentially stuck the blockchain in the source code by putting 'checkpoints' in the code. This protects their coins. The developers are the actual central blockchain authority. If they changed the checkpoints, they would fork the whole chain. The chain is essentially in the source code.