Let me rephrase. I don't think it was set in stone Rocco was going to die. I called it a suicide mission because it was going to be extremely difficult to kill Roth at the airport while he was taken into custody and get away with it. I think Rocco offered to do the job in order to redeem himself to Michael. And Michael allowed him to do it for that very reason and because of this among other things Rocco had become dispensable.

This is my canonical explanation. In reality Rocco, as well as Willi Cicci were supposed to be still around in GF III.