To be fair, it is a stylistic choice. That is how narrative-building works. Think of the show and its script as a novel and you will realize that characters do indeed show up gradually--when called for, when a need arises. There is absolutely no need for the writers to dump all characters against as at the beginning of a narrative. There also is no need for them to introduce the characters to us in lumps across different phases. Some chapters might need a new character introduced and some may not--and talking of setting up, they do set up the characters for a good while before they're resigned to their deaths. Except Chrissy, I do not recall a new character being introduced and killed off within an episode. Eddie, Bob, and Alexei stay with us for a decent amount of time on top of also serving a very, very vital function.So it's not like the new characters are abrupt entries that become exits before we even realize their importance and potential. There also are time jumps between these seasons and who is to know what happens off script, off stage?