Did I touch a never there?
So I guess you were typing aggressively when you did this?
I'm just going to say what's on my mind with this one, or based on what I
Setsuka-Relied heavily on the concepts of revenging her master and "Mitsurugi" as her plot device. That's the "literal" reason as to why she is in the game and the storyline. Both her debut and SCIV. She didn't care at all about the weapons like the rest of the characters do, nor did she care about the other characters in the roster. Mitsurugi may have spent most of his game appearances trying to find challenging opponents, but he wanted Soul Edge as a way to make himself more powerful, just like Yun-Seong was with obtaining them so he could win the Korean War (Hwang's motive at first in SE). Apparently Mitsurugi changes his mind from time to time. Either way, after Setsuka defeats Mitsurugi and has no desire of avenging her master, her plot device is gone, and if she returned, she would probably just be there.
Yes without Sophitia, Patroklos and Pyrrha would not be born, but without Tira, and Sophitia's death, neither of these two would have a plot device, or a motivation to obtain these swords, unless PS somehow found a way to do so. Patroklos is the one affected by it the most, because that was his whole reason for wanting to seek "justice". To avenge his mother against the malfested. He knew that SC would be responsible at stopping it.
Either way, you're pretty much saying "without "a", there would be no "b"". That is an obvious yes by the examples you listed. Well, if Mitsurugi didn't kill her master, she wouldn't even be seen in this game. Of course they may have given her something different when she debuted, but this was what PS came up with, and unfortunately they have to run with that reflecting her as a character. You're using rather vague cause-and-effects as your argument which in itself, doesn't support what you are REALLY trying to say, which was Setsuka being in SCV and not Alpha Patroklos, or Alpha Patroklos even using her style in the first place.
As much as it was a horrible decision, after looking at the story mode,
potential on Pyrrha being the only S&S user, his moveset, and the backstory, I've grown to accept Alpha Patroklos.