Sometimes peace comes with acceptance of things you cannot change. I doubt there is any way to reason with your parents, it's not about logic or reason, your parents are under cult mind control. You cannot counteract thirty years of indoctrination with a conversation or two. They have been trained to be fearful, on a very deep level, if challenged with different viewpoints, especially from former members. Fear like that is very powerful.
Even if you could convince them, I am not sure your parents would be better off, change is hard, especially when you are older, it's a way of life for them. I know that's hard, my mother died while she was shunning me, but I take comfort in the fact that she only did it because she felt she had to.
If you choose to reason with them, you might remind them of this, the July 2009 Awake p29:
“No one should be forced to worship in a way that he finds unacceptable or be made to choose between his beliefs and his family.”
Also, if you consider the scripture used to justify disfellowshipping, you can see it actually is talking about something very different than what is practiced by the Jehovah's Witnesses.
I cor 5:11" actually, I wrote to you not to associate with any so-called brother if he is an immoral person, or covetous, or an idolater, or a reviler, or a drunkard, or a swindler-- not even to eat with such a one."
Notice that it talks about someone who is a so-called brother. While that might have applied to you previously, it hardly applies now. It also says nothing about a judicial committee, any formal announcement, or parents shunning children even when they are no longer a part of the congregation. It simply means that if you know a person who pretends to be a Christian, who does these things, you don't talk to them.
"12For what have I to do with judging outsiders? Do you not judge those who are within the church?…"
The intent was obviously to keep the congregation clean, so what purpose does it serve now? What point is there to shun someone who is not even in the congregation anymore? You minght tell them that that many people have committed suicide after being disfellowshipped, and that many people have been disfelowshipped even though they had done nothing wrong. Also many people have repented and attended meetings for months or years but were never reinstated. It is also evident that elders, or the family of elders are treated more eminently that others.
You might remind them of the story of the prodigal son, who was warmly welcomed back, even after many year of a sinful life. His didn't greet him with a judicial committee, he didn't interrogate him, he didn't judge him, he threw his arms around him and had a feast. Also remind them of the story of the adulterous woman, where Jesus said that whoever is without sin should cast the first stone.