User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-27006668-20150927144953/@comment-24834479-20150928221442

Cain could literally not be killed, so he would have won in a fight to the death with Michael, as he had something that could kill him. Who said that he couldn't kill a regular angel anyways? Michael isn't strong enough to remove the mark anyways.

If the plans hadn't changed for Castiels Death, he would have either been killed by Alastair (with an angel blade), or not been brought back after Lucifer killed him.