> don't let 3rd party apps replace siri / "hey google"
I think it's reasonable that they don't let you replace the logic behind their branding – it would be quite a serious issue if ChatGPT started giving offensive results when you ask "Hey Siri".
Whether they should allow replacing the voice assistant entirely is an interesting question. On Android OEMs can of course do this, and have done so with mixed success, see "Bixby".
The issue with LLMs is that they are slow. People expect a response immediately, so much so that Amazon Alexa built much of its popularity as a kitchen timer over the alternative because it's just fast.
I think it's a much more reasonable argument that the reason Alexa, Siri, Google Assistant, Bixby, and Cortana (not sure if that exists anymore?) don't use LLMs is not that Amazon/Apple/Google/Samsung/Microsoft are asleep at the wheel, but that LLMs are not suitable yet. I'm sure this is coming, but I'm not surprised none of them have it yet.