If we only made decisions based on whether we could absolutely be sure of something with 100% certainty, we would all be paralyzed. Until a fully open-source phone is released that is comparable in quality to an iPhone, we can look at all of the available data and make reasonable decisions.
Aside from Apple’s strong stance and history of protecting the privacy of users, we also have security researchers MITM’ing the traffic sent from iOS devices to Apple, and every jailbreak gives an opportunity to look deeper; if they were acting badly, someone would find that out soon enough.
I agree that asking for 100% guarantees isn't useful. Realistically, the danger with Apple is that its absolute power over what regular users can install on their iOS devices is easily exploited by governments.
If some surveillance happy government bans some VPN software then Apple is going to enforce that ban much more thoroughly than Google or Microsoft ever could.
Aside from Apple’s strong stance and history of protecting the privacy of users, we also have security researchers MITM’ing the traffic sent from iOS devices to Apple, and every jailbreak gives an opportunity to look deeper; if they were acting badly, someone would find that out soon enough.