I won’t speak for the author, but I think everyone complaining about the girls disappearing is missing a pretty obvious set up, especially if you’re a wn reader.
Shirou is heading to the main continent where an on going war that EVERY love interest (aside from Sierra) we’ve seen have some sort of stake in, even some that haven’t been introduced yet. Shirou is also soon getting the means to directly intervene in that war, both combat wise and even in standing among the warring parties. Sierra can also show up at really any point in the story and likely will. There’s no reason to doubt that which ever girl you like won’t show up again and subsequently run through Shirou.
Shirou is heading to the main continent where an on going war that EVERY love interest (aside from Sierra) we’ve seen have some sort of stake in, even some that haven’t been introduced yet. Shirou is also soon getting the means to directly intervene in that war, both combat wise and even in standing among the warring parties. Sierra can also show up at really any point in the story and likely will. There’s no reason to doubt that which ever girl you like won’t show up again and subsequently run through Shirou.