That Siri went from useful to far less useful had more to do with the aim to push products at you rather than actually accomplishing the task you set for Siri. If Apple actually delivers an assistant that works locally, doesn't make me the product, and generally makes it easier to accomplish my tasks, then that's a product worth paying for.
When anyone asks "who benefits from 'AI'?" the answer is almost invariably "the people running the AI." Microsoft and OpenAI get more user data, and subscriptions. Google gets another vehicle for attention-injection. But if I run Vicuna or Alpaca (or some eventual equivalent) on my hardware, I can ensure I get what I need, and that there's much less hijacking of my intentions.
So Microsoft, if you're listening: I don't want Bing Chat search, I want Cortana Local.
There are definite frustrations, mostly around playing music. Around 5% of the time, Siri will play the wrong album or artist because the artist name sounds like some other album name, or vice versa. I wish, here, that it used my Music playback history to figure out which one I meant
Once you have the intents parsing, it should be just a matter of throwing man power at it and giving it better intents.
Yes, I have experience with building on top of such a system.
It would be easy to write Siri again and make it a hundred times better, if you could start all over and only write the core features, and not have to validate against the whole product/feature matrix.
The problem with the rewrite of course would be that you won't be able to deliver that minimal viable product any more and you will have 10 years worth of product requirements and user expectations that you MUST hit for the 1.0 release (which must be a 1.0 and not an 0.1).
I've worked on lots of "simple" and "not rocket science" systems that were 10-years old, and it is always incredibly difficult due to the state of the code, the lack of resources, and the organizational inertia.