A truly heartwarming moment. Because of the differences between the Campione!verse and the Nasuverse, Ilya no longer has a death sentence looming over her. She is starting to age.
It's actually less of the Campione!verse and more because of Shirou's Hero's Bride Authority, though it does not diminish the heartwarming value.
Shirou has been through so much stress and is thinking about Sakura. So what does he do? He writes letters to her in his journal and swears that one day, she will read them.
And she does, sometimes in the future. Which prompts her to force her big sister to let her meet Zelretch in order to learn Kaleidoscope so that she can search for him and bring him back.
The reason why Shirou was able to turn against Venus after she stole the entirety of his heart with her Heart Thief Authority. With it, she stole every shred of love he had for anything, every single bit, even his love for Sakura, and yet in the end he still chose Sakura.
Venus: "But . . . you love me."
Shirou "With all my heart. But I chose her over everything . . . even my own heart."
from Venus's pov however, along with her last thoughts, it can double as a tearjerker.