Lilmod,
I don’t think you (or I) can say what level of gadlus Rabbi Avigdor Miller achieved. It is unwise to simply declare that he was not in the same category as Rav Moshe Feinstein.
Everybody agrees that there are cases where divorce is necessary and (I think) everybody would agree that there are nebach divorces that occur today that are unnecessary and should not occur. Ones position on the actual percentages of these will depend on what situations you believe divorce can be justified.
It is my personal opinion that, when there are children in the home, divorce is wrong unless: 1) One parent is abusive (to the children or to the other parent; 2) One parent has become a drug or alchohol addict; 3) a parent has become mentally ill; 4) One parent has gone off the derech; or 5) One parent was unfaithful.
My understanding of Rabbi Avigdor Miller’s position is along the same lines.