I think it's a combination of three things:

1) AI threatens to take-over how you use your phone, it threatens to reduce apps to an API that it will use on your behalf so you don't use the apps yourself

2) By doing that it commoditizes the hardware because the software experience is virtually identical across platforms, you say make a dinner reservation and it doesn't matter what calendar you use, what restaurant app etc

3) Apple is no longer assured to be able to gatekeep or ban these things so if they aren't producing the most useful or entrenched assistant someone else could become people's primary interface for iPhones

There's a lot of parallel with "super apps" -

> Apple’s fear of super apps is based on first-hand experience with enormously popular super apps in Asia. Apple does not want U.S. companies and U.S. users to benefit from similar innovations. For example, in a Board of Directors presentation, Apple highlighted the “[u]ndifferentiated user experience on [a] super platform” as a “major headwind” to growing iPhone sales in countries with popular super apps due to the “[l]ow stickiness” and “[l]ow switching cost.” For the same reasons, a super app created by a U.S. company would pose a similar threat to Apple’s smartphone dominance in the United States. Apple noted as a risk in 2017 that a potential super app created by a specific U.S. company would “replace[ ] usage of native OS and apps resulting in commoditization of smartphone hardware.”

https://storage.courtlistener.com/recap/gov.uscourts.njd.544...