So, someone studying it now comes to a different conclusion than the debaters of the day, therefore it is valid? Based on what authority?
Protestants cannot get past the other fact regarding the ECF. Whenever a question or dispute arose, Rome had the final decision. Roma locuta est. Causa finita est - Rome has spoken. The cause is finished. It’s not so much the defense of the practice they should be looking for. It’s the fact that the practice was used, period.
It was the successor to Peter who held the “Keys to the Kingdom”. Those keys didn’t disappear with the death of Peter. It is not the location of Rome that is supreme. The Pope has had to leave the Vatican on occasion over the centuries to minister from elsewhere due to different threats. That’s not much of a worry nowadays.
Protestants should asked themselves why disputes were referred to Rome, and why Rome’s decision was accepted afterward. Those who did not accept it were those who believed Rome reached the wrong decision, but they were perfectly fine with deferring to Rome when they thought something would be decided in their favor. How ironic. This is happening today with liberals rejecting Humane Vitae, and conservatives rejecting Vatican II. As you can see, you do even need to leave the Church to reject Rome, but why would you want to?