Siri is already transitioning from an intent-based NLU system to an LLM.

In iOS 18.1 (on iPhone 15+) Siri is part intent-based, part on-device "Apple Intelligence" small LLM, and in iOS 18.2 it also supports off-device ChatGPT.

This year Siri 2.0 is expected to ditch the legacy intent-based system and instead use just the small on-device Apple Intelligence LLM plus (opt-in) off-device Gemini (running in some private cloud).