The beautiful thing about River Song is that it doesn’t matter.
While Clara’s world shatters into pieces when her Doctor swaps the bow tie for the cardigan, River does barely notice what face he happens to be wearing.
She doesn’t have ‘a’ Doctor.
They are all hers.
You can’t see me, can you? You look at me, and you can’t see me. - Deep Breath
River is a Time Lady, through and through, and once she learns her odd companion she is having dinner with on a soon-to-be-crashing starship is really her Doctor stuck in an unlicensed regeneration cycle, she does what Twelve needed Clara to do in Deep Breath: she sees him as if the new face - the new everything wasn’t there.
It’s with River Song that we truly get the feeling that yes - that man with those many faces and silly clothes is always the same. He is the one thing in her life that doesn’t change. He is her touchstone that’s been there from the very beginning, and will be there till the very end.
The moment she spots him in the abandoned Library, she expects him to pick matters up just where they left off, like they always do. It’s a statement to their relationship that she knows him well enough to make them click - to make him say his lines without knowing the rules. Without knowing there are rules. She’s become so good at it, that it takes her far too long to realise that this time she’s gone too far back in time, that really he has no idea who she is.
In the end she does what she was forbidden to do during their last big adventure: she saves his life, and all their adventures along with it. Time can be rewritten - she has done it more often than history books and the Doctor care to count. But she won’t allow him to change that one moment, no matter how much she knows it must hurt him.
He doesn’t want that sacrifice. He cannot take it. She dies but he still finds a way to save her, to cheat death one last time, even if everything about his solution feels all too much like a defeat.
That’s why he waits long enough for them to become the stuff of legends, why he just cannot face living up to his promise. Not taking her to Darillium is his way of making sure he’d see her again, that they would have one last adventure. But if Clara has taught him one lesson then it’s that
people are entitled to a past, a death, a happy end. And River’s death in the library is something he just cannot change. He has seen it happen. He outlived his regeneration cycle biding for a bit more time; he witnessed his wife, who knows all his faces, not recognise him as he had escaped death too often. So, he does what
his past faces weren’t able to and shows his River the Singing
Towers. It breaks his hearts but what else can he do?
Because everybody knows that everybody dies and nobody knows it like the Doctor.