"let the user install their own certs should not exist"
are you sure that this is not opening to more attack vector ?????
You can even install your own certs into the golden cage (also known as iPhones).
Users aren't supposed to be considered an attack vector. I don't care what the advertisers think.
Which is better - two attack vectors that can be mitigated by an expert, or one attack vector that can't be mitigated by anyone?
Yes. We're giving the user the ability to protect themselves from the vendor.
Your logic only makes sense if the user is your enemy.