Half untrue -- Apple allows you to use third-party JS engines on iOS. I've seen SpiderMonkey and Jurrasic (C#) used on iOS. What they don't allow third parties

Half untrue -- Apple allows you to use third-party JS engines on iOS. I've seen SpiderMonkey and Jurrasic (C#) used on iOS. What they don't allow third parties to do is JIT compile. The "execute writeable pages" entitlement is only allowed to Safari and in iOS 8 WKWebViews. So your bigger point stands -- third party browsers all need to use a single JS implementation if they want to have the speed of a JIT. I think I prefer a single trusted implementation by Apple to N implementations by whoever wants to give it a shot.