What Mikasa is seeing...
Armin's struggling in the penultimate physical head to head against Eren.
Reiner, Pieck & Annie are all on the verge of death getting mangled by their own blood, family and sworn brothers.
Everyone she cares about and loves are dying in front of her.
This is her Last Stand and she is the only one with full physical powers & a hardened resolve who is capable of saving everyone.
From the looks of it, Ymir has finally found the truest, purest meaning of life: love. Through Mikasa's pure, selfless action, persevering through the pain of her unattainable dream; of letting go, Ymir is finally fulfilled and at peace, apparent from the smile on her face. She is seeing that humanity deserves the life it's been given.
Mikasa chooses to sacrifice the person she loves the most to save humanity and the other people she also loves & cares about from dying - that is the True Ending & the final route that completes Ymir's story.
It was a tremendous price to pay but Ymir has finally seen the beauty in this cruel world through Mikasa's eyes.
EDIT: Mikasa seizes control and fought against the very same person she loves and who has influence over her will, dreams and desires. She did what Ymir wasn't able to do against the first King Fritz despite this tremendous power that she wields. Mikasa is truly free. Ymir is witnessing it. Apart from Armin, Ymir has also been observing Mikasa in 135 but this deserves its own post!
Allow this poor, unfortunate girl the panel she needs for her closure & the space to say her farewell to the person she has built a world for in her heart and in her mind for the last 10 years.
