Re: Safari and Internet Explorer
"Adobe Flash was banned from iOS because its wonderful rendering engine killed off the battery. That's the kind of world you can look forward to with arbitrary browser engine installation."
Lots of apps kill the battery. That's not against the rules. You can either discard them as bad apps or decide that you like them even though they won't let you run very long. That's not Apple's decision to make, and they repeatedly don't make it when it's a different kind of app. They didn't like Flash because: A) Adobe didn't port applications to OS X until after a long delay and Steve Jobs held a grudge, B) it killed the battery and they didn't like it, and C) they didn't like the structure and wanted to encourage adoption of other alternatives. I agree with their reasons B and C, but that doesn't mean it should be their decision what I'm allowed to run on my hardware.
"There *are* Apple competitor apps on the App Store, for example numerous email clients, so the "nothing that competes with something we do" policy is all in your fevered mind."
They have banned apps which compete with other parts of their system repeatedly. See the screen time feature and all the apps which did it and got banned for another example. So they didn't bother when it was a mail client. That doesn't mean they always allow it. They don't.
"WebKit is a core component shared by multiple apps on iOS."
Correct. That's just fine. Any app that wants to use it can. No argument here.
"If Apple allowed arbitrary browsers to have full control of the networking stack"
They wouldn't. Same access as any other app.
"and rendering capabilities,"
Sorry? "Full control of rendering capabilities"? What does that even mean?
"you'd end up with browsers like full-fat Chrome on iOS that use decidedly privacy-hostile web standards."
Agreed. I wouldn't be installing it.
"Apple can't permit that at the same time as advertising that they protect your privacy."
They could ban any app that tried under their privacy rules, not the rendering engine rules. You don't have to ban specific kinds of apps if you can just say that tracking without permission is forbidden whatever app is used to do it.