Respectfully, you're slicing and dicing the prophecy.
Isaiah 9:6 NIV:
For to us a child is born, to us a son is given, and the government will be on his shoulders. And he will be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace.
There are 4 titles conferred onto the target of this prophecy not just "Prince of Peace". If you go around the world asking people who is "wonderful counselor" will the consensus among atheists be your messiah? What about "Mighty God"? will millions of atheists apply this label to your guy?
Even if you're right about Prince of Peace it's only 25% of the prophecy. And this ignores the previous sentence "the governement will be on his shoulders" which is clearly not JC. At least not yet. So you can't claim this prophecy fulfilled until *all* the criteria are met. No cherry picking individual elements from the prophecy, please.