Once somebody has more computing power than everybody else put together, they should go about 1,000 blocks back and try to build a new chain building off that, in secret. (This would take a week.) However their chain will not include any of the transactions from the last 1,000 legitimate blocks. Once it becomes longer than the existing chain, they publish it and instantaneously a whole week of transactions un-happen, i.e. they are reversed and the money (including mining fees) returns to the hands of the original owner, whereas the 50,000 BTC legitimately mined disappear, and the new 50,000 BTC generated are owned by one bitcoin address, the attacker's.