I believe that's true - if you can get ahold of someone's private keys, you have free access not only to their money but also to their money's transaction histo

I believe that's true - if you can get ahold of someone's private keys, you have free access not only to their money but also to their money's transaction history. The security and anonymity of the system is predicated on the person wanting to be anonymous being able to keep their private keys private. However, private keys are easy to strongly encrypt, typically don't leave your own machine, and can be safeguarded in many other ways. So in practice, they are impossible to lift from a sophisticated user without performing an equally sophisticated attack of deception, for example an evil maid attack with specialized hardware/software.