If the other browsers could implement their engines, you would still have the option not to install them. So it’s okay for user who (in theory) installed a 3rd

If the other browsers could implement their engines, you would still have the option not to install them. So it’s okay for user who (in theory) installed a 3rd party browser on iOS to have a worse experience and potentially make themselves vulnerable to malware? That’s exactly what Apple doesn’t want.