EDIT: The same thing happens to Alistair if he comes out of the Fade, meaning that the Warden will never get a happy ending with Alistair if he remained a Warden as he either dies in the Fade or vanishes.
Well, let's not say NEVER. The thing with the Wardens hasn't been resolved at the end of DA:I, yet another dangling plot thread for DLC or maybe DA4. Besides, characters vanish and then reappear all the time in Thedas.
On a more selfish note, I romanced Warden!Alistair, Anders, and Solas. My Hawke is dead, Solas has all the ominous foreshadowing, so goddamnit I would like at least one of my canon DA couples to get a happy ending
I really know how to pick 'em. At least I went with Garrus for ME.