It really isn't just Siri. Apple intelligence is way more than that. For example, image playgrounds, visual intelligence, genmoji, shortcuts etc.
Apple Intelligence is the personal intelligence system that puts powerful generative models right at the core of your iPhone, iPad, and Mac and powers incredible new features to help users communicate, work, and express themselves.
developer.apple.com
So disabling Siri will not free up the space used by the models or disable all of apple intelligence.
oh and Apple ARE expanding to run on Googles cloud infrastructure.
Alongside the next generation of Apple Intelligence, today we’re expanding Private Cloud Compute (PCC) beyond Apple’s data centers. When Apple introduced Private Cloud Compute in 2024, we defined a new frontier for private AI inference, extending the security and privacy of Apple devices into...
security.apple.com
...but they are claiming separation.