Honestly, I’ve seen a few different takes on this, and I think some folks mix up the routes. In the original 'Tsukihime', the true ending depends on which heroine path you’re on. Most people think of the Arcueid route as the main one, right? In that good ending, after Shiki defeats Roa, Arcueid decides to go back to sleep because her bloodlust is too dangerous. She tells Shiki to live a normal life, and they part ways at dawn. But he keeps his promise to remember her forever. The last scene is just Shiki walking home alone as the sun comes up, and he says something like, 'I will never forget you.' It’ll wreck you if you’re invested in them as a couple. I liked it, but it felt abrupt, like there should be one more scene of him years later or something. The manga adaptation actually expands on it a bit more.
Sometimes I wonder if the 'good' ending is actually sadder than some of the bad ends. At least in the bad ends, the tragedy is immediate and dramatic. Here, it’s just this quiet, lingering ache. You’re left with the feeling that Shiki’s 'normal life' will always have this huge, supernatural-shaped hole in it. Makes rereads of the earlier, lighter moments hit different.