They just launched "Private Cloud Compute" with much fanfare to enable server-side LLM processing, so between that and the fact that Siri has been server-based for most of its existence (local processing is fairly new), I don't think that's their main constraint at this point.

That said, "Private Cloud Compute" does run on proprietary Apple hardware, so availability might be a concern (assuming they don't want to start charging for it).