The situation here is you can only track the addresses you know about, and you can never (with one exception[1]) be sure you know all of someone's addresses.  That means you can never determine the maximum number of bitcoins someone has if they use key rotation.

I'm not against fully explaining the risks, but I think the purpose of this section is to get developers to use key rotation.  This section gives them two strong reasons: more financial privacy and more forward security.  If we explain how that additional financial privacy isn't effective against a determined attacker---which almost no real-world financial privacy is---we risk making key rotation look more "optional" and less "highly recommended".

I suggest we keep the current form of the text, although I'm still open to other suggestions.

[1] The exception is if you know the owner of every other UTXO on the network, which isn't realistic.  It's so unrealistic that I didn't think it was worth mentioning in the text.