i guess the shirunin process is a promotional exam for established professional ninjas? disposing of a girl one has seen grow into adulthood alongside a comrade theyve spent years forming a bond with is probably a good test for the kind of cutthroat survival instinct that makes one well suited for ascending the ranks of NIN, hence why directly killing a shirunin, regardless of the context, nets a higher score for kuonichi than poisoning.
that lends some context to toichi's pessimistic view of kuonichi, at every stage of ninja life it seems their fate is to be used as cannon fodder for the men around them. even the highest ranking kuonichi can only be a wet nurse to their old fogey boss. but that means that suzuki's old man fetish puts her head and shoulders ahead of her peers when it comes to her passion for her work. meanwhile onikobe, who's ability to kill two professional ninja's as a student was a testament to her sheer talent, was probably the sort who'd be unwilling to play those kinds of games to ascend the corporate ladder, and would instead end up wasting away in a dead end position while the people she'd graduated alongside either surpassed her or got higher ranking positions in different organizations.
also saruta and nekohira looking down on shirunin is probably foreshadowing for their professional prospects. like nobody was going to ask yall, lmao
edit: i wonder if igarashi ever really wanted to get promoted or if he was always just in it for the love of the game (killing young women and girls)