Of course it's one of discipline. Immutable teachings cannot be changed, nor can exceptions be made to them. Both of those things have happened to the celibacy discipline.
In addition, as traditio itself points out, the Eastern Rites have "mitigated" the practice, something which is simply not possible with divine positive law.
Well, how could priestly celibacy last for so many years and the number of priests not go into a dangerous level, until the latter half of the 20th century. All the other years are proof that celibacy worked.
You might well be right, but the question has not been definitively answered.
There is good reason (i.e., the testimony of certain Church Fathers, the proceeding of certain Church Councils) to believe that celibacy has been regarded as of Apostolic origin. Married men have been ordained from the time of the Last Supper, but that is not the point. A body of evidence (which, I admit, is contradicted by other evidence) exists to suggest that married men were expected after their ordination to live as brother and sister with their wives. Stefan Heid (who is neither a Lefevbrist nor a sedevacantist) maintains as much in his recent study published by Ignatius Press.
This may well be as suspect as St. Epiphanius's insistence that the Lenten fast was of Apostolic origin. But that is neither here nor there. The point I am making is that, barring a pronouncement from the highest authority, we can neither dismiss nor insist upon one interpretation or the other.
I don't see how the Second Vatican Council's resurrection of the permanent diaconate is relevant to this discussion, since this Council's pastoral proceedings have no dogmatic force. (Neither, I admit, did the disciplinary canon of the Council of Nicea which enjoined perfect continence upon all who had taken Holy Orders.)
But the question has not been disposed of.