Given this it's really a shame that Apple does not allow other browser engines on iOS. Would be a prime opportunity for competition to raise the overall quality level.
WebKit is a core part of iOS; lots of APIs use it for lots of things. You just can't swap it out for something else without breaking things.
Lets not forget the security and power usage issues as well; the last thing iPhone users would want is a rendering engine that wasn't optimized for the hardware and software draining their batteries.