WMG: Due to her remaining connection to Akasha/the-other-side/whatever after HF true and her receiving almost infinite prana from there, Sakura retains her super-regeneration at the end of HF true. This makes her "immortal" since she also doesn't age because of the virtualy infinite prana. Since she has sex with shirou on a daily basis, where she shares that prana with him, he is also "immortal". Therefor Shirou and Sakura spent the rest of eternety (or at least as long as Gaia exists)
fucking like rabbits together!