The blockchain is a system of verification not trust. But once a transaction is confirmed in the blockchain it is still possible for a pool with a large enough percentage to attempt to remove it. The more confirmation, the more the block gets buried under other confirmed blocks which were created with proof of work and the more computing power a malicious power will need to attempt a reorg.

We're talking in terms of a humongous amount of money and the obvious risk to your reputation for no guarantee that the malicious attempt will be successful. It's a no brainier to work honestly on the network.