What exactly are you proposing as an alternative here? Completely ignoring what's good for Rin and only thinking about what's good for Shirou there are four possible approaches
I... have to propose an alternative? Why is that? At the moment Rin is trying to win the war, which involves killing Shirou, Taiga, Illya, Sakura and now possibly Ayako. Oh, sure, it's 'eliminating', but take one look at Saber's arsenal and mindset and you tell me how much thought is being put in preserving people's lives versus achieving the goal.
- Stop Rin by killing her. This is the easiest most direct way to neutralize the threat she poses to the public. It also forces Shirou who's already been traumatized enough by this nightmare of a situation to bear the burden of killing his own classmate, irrevocably strains his relationship with Sakura and Sakura's own chances at happiness, and sends him directly down the path to becoming Kiritsugu. If we're prioritizing Shirou's well-being this should be the last option we consider when all else has failed
That's a possibility, yes, but as for it being 'the last', I can think of two options that come before it. Letting innocents die, and letting any of our friends/allies/camp followers come to irrepairable harm. If it comes to that, Rin dies, and if it even comes to
risking that, there is a high chance Rin dies just so it is avoided.
And it will come to risking that. It's not the vote button I would smash with wild abandon, but I am not letting people be killed off so that Sakura could be happy drinking tea with their murderer in the aftermath, and I find Shirou's peace of mind is a weak reason for gambling with their lives.
I'd add 'dying ourselves' to that list, but with the way the quest mechanic works, it's a complete non-issue and does not warrant a consideration from the players.
Years spent on this site observing magical girl quests have inured me to these situations. A tragic, broken villain's sob story is revealed (because every QM worth their salt won't write a villain without a motivation), and the thread goes "aww, this tall-dark-and-mysterious is so cute and broken, let's hug the evil out of her", and should the genre tropes fail and the expected happen, the same thread would be clamoring for her blood because she hurt the characters people really care about.
3. Force Rin's surrender at gunpoint. Completely crushing Rin's ability to fight back, forcing her surrender, and then imprisoning her to prevent further damage is possible. It is also easier said than done. Assassinating Rin would be one thing (Mata Hari already could have and chose not to) but defeating her without killing her is dramatically harder. Rin's a prodigy of a mage, Salter with Rin as a mana source is enormously strong, and they're both excellent tacticians who know when to retreat. Actually beating them in a straight fight with the servants we have available is going to require an enormous amount of careful planning, could easily cause collateral damage, and will never be a sure thing. That being said this option would stop Rin from endangering the public while preserving Shirou's conscience
Yes, otherwise known as 'killing the Servant'. This is Shirou's go-to option, now that 'not fighting at all' has failed. Dangerous and oftentimes stupidly suicidal compared to more expedient solutions, it is the perfect fit for the character. I expect this to be the thread's choice in absense of stronger motivations.
4. Talk Rin down. Like I said in my last post this is doable. Rin already indirectly has blood on her hands but she's wavering and has yet to pass the point of no return. Giving her the opportunity to get off this train before she does solves things in the way that will make both Shirou and Sakura happiest
Yes, this is
theoretically possible.
However, what I see in the Interlude is not wavering. It's regret, yes, but also a resolution to go through with it now that she's come that far. Before she is a Young Girl, she is a Magus and a Master. She'll cry her eyes out, wipe the tears, dust herself off, and will continue on the path she set for herself. Because it was not a misstep, but a logical development.
Before she killed a person in collateral, she led her Servant to damnation, and before that, she resolved to kill a classmate for being a rival. Each step is easier than the one before it, and the next one -- ignoring the notion of the collateral -- will come to her even easier and with minimal heartbreak once she is done crying. I am not particularly sure why it falls on us to be her therapist and conscience, but the time where she'd listen to words alone had been at the start of that road. Now she will have her pride and sunken costs to drive her forward.
And she has positive feedback about her decisions, too. I can't see a way we get through to Rin without removing her Servant out of the picture first. There is no plausible scenario under which a thing like Saber is being talked into standing down and turning down victory she sacrificed so much for. Wherever Rin is on her way to the bottom, Saber is two steps ahead, and is lending her an ever-helping hand.
Still, I'd love to see the thread reaction if it were Rin we'd come to talk to instead of Illya. "Alright, you and me then. Come alone." Would we do this? Ah, what am I saying, of course we'd do this! "You and that meguka over there," then. Now it's not so simple, is it?
I am just having trouble imagining the circumstances under which we'd be able to hold a conversation that would change anyone's mind about anything.
So my personal bet is on 3, keeping 1 in reserve.