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.
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.