While this post raises legitimate points, I don't think Apple disallows other browser engines or default apps for fear that they maybe more insecure.
Speaking from a business perspective, it makes sense for Apple (if not for users) to force Safari and Mail.app. Otherwise, Chrome and Gmail will just overtake the phone.
This will be bad for Apple from a strategic and PR perspective. Gmail and Chrome would have no incentive to improve battery life (indeed Chrome did not improve battery performance till 2017 on Macbooks and Chrome 76 has again destroyed battery life on my macbook) and Apple's privacy-as-a-selling-feature would go out of the window because - media, people and their willful ignorance of nuance and context. Not to mention, how your product roadmap would be influenced by another company simply because it has a much larger user base.
On the security aspect, I am not a fan of how media has been posturing this as - "iphones are complete security nightmares." Any person who has followed security will astutely note that iOS garners the same attention which Windows did a decade ago. At that time, it did not mean that macOS or Linux were inherently safer, just that they did not have the attention that Windows commanded. Black market prices of exploits are not a good proxy for how secure is a system. It just indicates which operating system has the most attention of hackers. It will not be a surprise to me if 2-3 years later Android finds itself in a similar position as iOS is in now. Not to say, that the exploit surface on Android can be way more bigger because of the number of parties involved - OEM and the carrier.
I think one huge reason for not allowing other browser engines is because to get performant javascript, a JIT javascript engine is necessary, which in turn requires access to mark memory pages executable after being writable, which is a code entitlement that Apple will never ever hand out to third party developers (because it allows bypassing app store review and is also a huge liability security-wise).
If third party apps had access to map dynamic memory as executable, we would see thousands of third party "appstores" or "emulator/piracy launchers" that could simply download random unvetted executables from the internet, perhaps hidden in inconspicuous decoy "flashlight apps".
If they allowed third party browser engines without JIT, everyone would install Chrome and then complain that iOS benchmarks incredibly bad compared to Android.
Speaking from a business perspective, it makes sense for Apple (if not for users) to force Safari and Mail.app. Otherwise, Chrome and Gmail will just overtake the phone.
This will be bad for Apple from a strategic and PR perspective. Gmail and Chrome would have no incentive to improve battery life (indeed Chrome did not improve battery performance till 2017 on Macbooks and Chrome 76 has again destroyed battery life on my macbook) and Apple's privacy-as-a-selling-feature would go out of the window because - media, people and their willful ignorance of nuance and context. Not to mention, how your product roadmap would be influenced by another company simply because it has a much larger user base.
On the security aspect, I am not a fan of how media has been posturing this as - "iphones are complete security nightmares." Any person who has followed security will astutely note that iOS garners the same attention which Windows did a decade ago. At that time, it did not mean that macOS or Linux were inherently safer, just that they did not have the attention that Windows commanded. Black market prices of exploits are not a good proxy for how secure is a system. It just indicates which operating system has the most attention of hackers. It will not be a surprise to me if 2-3 years later Android finds itself in a similar position as iOS is in now. Not to say, that the exploit surface on Android can be way more bigger because of the number of parties involved - OEM and the carrier.