It sounds like a low hanging fruit, but it's far from it.
You can make LLMs work _anecdotically_, but the brutal truth is they don't scale. Money-wise but more importantly for Apple - accuracy and reproducibility-wise.
Siri is dumb, but it's very predictibly dumb and it does those few primitive things it does reliably well with zero cost for apple.
90% of what I need Siri to do it does with fairly good accuracy.
Send a message, read messages, call X.