If you look at a lot of products out there that are basically what people want Siri to be (a truly integrated AI assistant), they either fall super flat (see rabbit r1) or are prohibitively expensive (see motion ai).

Additionally, I think another one people forget is that Apple positioned itself with their marketing as the last bastion of privacy (I'm emphasizing marketing, not reality here), which gets contradicted with AI's current public perception. There's also the issue of choosing whether to keep things in the cloud or on device.

One final major thing that I think Apple is considering: how could AI integration inadvertently cannibalize their app store profits? If they're able to offer AI integration that other calendar, todo, etc. apps can't, how will that affect a user's purchasing decision? If doing this would potentially eat into app store profits, then the next logical step is to create a system that allows developers to essentially have an API into the user's trained data, which is likely no small feat, especially if Apple is trying to obscure the data exposed to the app developer.