> It is not our experience that competing web browsers have typically offered enhanced privacy or security that would protect users as adequately as our WebKit protections.
Google pioneered the out-of-process architecture that Safari now uses, developed the Safe Browsing program that Safari also uses, drove the adoption of HTTPS, put pressure on misbehaving certificate authorities and shepherded certificate pinning and then Certificate Transparency, and found many vulnerabilities in WebKit through security research that Apple was not itself doing.
Moreover, on the desktop, Chrome and Firefox both have automatic update channels that allow them to push out security fixes much more rapidly than Apple's heavy OS updates. (On iOS, they would be limited by Apple's App Store approval process.)
All this to say, I'm skeptical of the suggestion that a Blink-powered Chrome for iOS would not "protect users as adequately as our WebKit protections."
One thing to note: JavaScriptCore has special powers on iOS in that it can allocate new blocks of executable memory during just-in-time compilation, greatly improving its performance. A third-party JavaScript engine would be much slower without this ability, but granting the capability to third-party engines would jeopardize the security architecture of iOS.