To allow users to query for anything? The index isn't to train and build a model, it's to behave as a search engine for their users. It would (or could) effectively replace Google for Apple users.

Imagine an updated Spotlight that would allow the user to enter any query, obtaining information from the internet, enriched with their local context/data.

LLM Siri is an entirely different concern than Apple potentially acquiring Perplexity. I view them as two wildly different initiatives.