Apple's ecosystem is made up of their devices and services.
You'll still need a device to use A.I. and you're still going to buy an Apple Watch for its fitness and health tracking features.
How is A.I. going to replace an Apple sevice like Apple Pay, Card, Savings, iCloud, News, Music, TV+, etc.? Is A.I. going to make people consume less content (news, music, video, games)?
You're telling me people will be using their iPhone less? People won't be browsing Facebook/Instagram, X (Twitter), TikTok, etc? People are going to be taking less pictures/videos and posting them online? People are going to give up being influencers and content creators? On the contrary, I think generative A.I. will see more people creating and editing content on their iPhones.