That's the trick. In some cases, you'd be correct and the thieves can get away with theft. In most cases though you are incorrect, the cost of getting that many ledgers to agree with the thieves is prohibitively high.
This theory underscores the importance of having many full nodes running on the network. If only a small number of people run validating nodes, the cost of committing some theft like this is substantially reduced. This is one of the biggest and most important arguments behind having small blocks instead of large blocks.
Large block supporters tend to think you only need a few full nodes to get immunity from these types of attacks, and small block supporters tend to believe that a small number of nodes is easy to compromise relative to the reward for doing so.