Ugh its not a story beat, we're literally in the afterlife, she has literally died 9 times or whatever. Killing her does what exactly? she dies properly and what? she turns up where? exactly where she is? in the afterlife? whats the point? oh now shes free of the eternal suffering that comes with being undead? good for her!
It has nothing to do with being the bigger man its about spite, taking from her what she always desired. Killing her literally rewards her, leaving her inhabiting a rotten corpse and forcing her into endless servitude is a far bigger punishment then letting her off.
People who want her dead completely miss the point, i get that all they really want is to have her out of the story and they dont really give a shit about the narrative but killing her is a reward not a punishment. Its the eternal rest she wanted since warcraft 3.
I get peoples anxiety about her showing back up too soon but shes out for at least 3 expansions, we had illidan return and that was handled pretty well, hes not chilling out at a tavern now hes Sargeras's jailor, he didnt come back next xpac, or the xpac after or the one after that, hes out of the story now for 3 consecutive expansions (just how it should be) and i expect sylvanas will be the same. When she does come back it would probably be some segue into another death themed expansion, which would be at least 4 expansions away surely.
Killing her just denies her the consequences of her actions, its an incredibly lazy end to her arc. Tyrande sentencing her to what would realistically be an eternity of punishment is very fitting. There is a LOT i hate about both of their stories but the end was actually a perfect fit.
As for anduin visiting, denying him access to her is a punishment to anduin not her, she didnt request his audience, he went to her. Visitors are allowed to visit prisoners in real life too, thats a stupid complaint, and for what its worth i think its stupid that he did at that point in that way, and i also think its stupid that khadgar visits the shadowlands too, its starting to feel like a tourist destination.