The number of dumb hot-takes in response to this article is staggering. The system as described so far is opt-in for anything sending your data outside of either the device or a carefully vetted and privacy-first cloud system that was designed by people far smarter than you to make sure that Apple could not get to your data. Apple knows that it is better for them to be unable to see this data and to be able to prove it to anyone holding a warrant.
Will there be a general on/off slider just like there is for Siri? Probably. Will more than 1% of the userbase turn it off? No.
Will I be surprised if, days before Apple Intelligence is released to the general public, it is declared that apps distributed via non-Apple app stores are not eligible for linking to these AI services (payment of the cloud services and OpenAI being funded by App Store revenue, naturally, and TANSTAAFL)? No, I will not.