> Because of security, browsers are a huge vector of attack.

That's not a valid reason. With per-app sandboxing. At worst a non-Apple native browser that escape the confines of the web page and be subject to app security.

That amounts to using "security" as an argument for not being allowed to do something with the reason as, "Because we're smart and you're not".

There is a sandbox, but to be a competitive third-party browser you also need something else:

Safari has a special permission (entitlement) that allows it to compile JavaScript to native code. This is what their JIT does.

This is something that no third-party app has ever gotten. No JIT means slow JavaScript. No ASM.js, no WebAssembly.

I have mixed feelings about giving third-party applications (browsers) this permission. Bugs can lead to remote code execution, can lead to sandbox escapes, can lead to compromised devices.

This is a really tough one. Specially for apps that load random content from the web.

Then I guess we need to get rid of all the alternate browsers on Android, Windows, Linux, etc because they are all just ticking time bombs waiting to destroy your device.

They are. But Apple was the only company smart enough to control that on their mobile platform BEFORE it became an issue.