Isaiah doesn't say we *might* have peace- he says we WILL have peace. There's no question or doubt. True peace is ours and it can only be obtained through a relationship with Jesus. Pure and simple. Us humans always overcomplicate everything.
By having faith that Jesus is the Son of God and our Savior, we can have peace - inner tranquility, calmness, peace of mind - in all things. He is responsible for everything good in our lives.