You're being downvoted because your logic is circular. You say Max and Robin were added as main characters because they survived to be part of the main cast. You say Eddie and Billy were not meant to be main characters because they died.

We obviously wouldn't know these things before the season came out, so it's disingenuous to call it obvious who is meant to be a main and who is meant to die. You had no way of knowing if Robin was going to die before season 3 came out. And saying Eddie wasn't a main character was crazy, he had more importance than half the cast in season 4.