You've hit the nail on the head. So many jailbreak exploits have been done because of Webkit exploits. Allowing other engines on the phone would make them lose control over their walled garden.
The evidence points that Safari's security is in fact worse than either Chrome/Edge or Firefox's so that's going to be a hard sell to even prove they are equivalent.
Intentional jailbreaking is a problem for Apple, not for Apple's users.