The thing is that you're talking to someone who thinks a character is completely useless in this game if they're not stuck eternally into a "fucked by NTR Chad" situation, which Suguha's entire plot in Part 2 has completely denied repeatedly, hell, she literally has become the most relevant character outside the main story because of this (And recently even easily eclipsed the likes of Kyouko and Liz easily plot-wise despite not having as many H-scenes). That guy is just too braindead to realize or seems to not have been playing the same game at all, honestly, especially if his main argument is that "Nobody cares about her" because lmao, 'nobody' period cared about her back when she was Hebishima slut/was under the NTR cliche plots, she only got actual fans around here beyond me and one other when she broke free of them and started her very out-of-nowhere new plot
The biggest issue this guy's logic you're trying to accept has, and most of what you and others have been saying really, is seeing Suguha's entire plot as "vanilla", which if it was then it WOULD be doomed to fail in a NTR game, but it's anything but and that's why her plot has all the hope to continue as it is...Suguha is basically crazy at this point, with several yandere undertones, raping her brother unconscious against his will, undermining his relationship in subtle ways, dropping threatening bits, etc.
Another massive difference is that, unlike others, the plot/Fujino ACTUALLY let her do this in the first place...if she was 'doomed for the cliche NTR plots', as some keep thinking, then she would never have been allowed to start raping Kirito in the first place rather than it become a regular thing, and we had the biggest example of this last update: Liz TRIED to do this, but the plot didn't even let her fully enjoy a kiss with Kirito, and she immediately ended up raped and with her first time stolen by Inoda right away thanks to several dumb coincidences overlapping into each other XP