My ending was really bad and sad so i wouldn't be surprised if he just gave up on life.
He lost pretty much everything :
His adopted daughter Ciri - died
Uncle Vesemir died ( he was like a father to him )
Triss and Yen left him
His Witcher friends left him
Vernon Rache and Bianca died
He chopped Keira's head
plus Geralt is getting too old for killing monster's
in my opinion Bad Ending is the Best way to end Witcher game, The revenge on that last Witch was too damn good.
How on earth did you managed to do all that?
But back to the point... My boyfriend had that sad ending too. He was certain that Geralt died. I'm not sure. It looks like he died, but I think that there is a reason for that little "backdoor" that he didn't. What if Geralt's story continues sometimes? Even if this supposed to be last part? What if the Wild Hunt ended but something else comes up?
What becomes to those good choices... Ciri is/was kinda difficult and stubborn person. Who knows what Ciri thinks is best for her? Like @dmcaldw said.
And the greatest thing of all... Although the ending seems horrible and sad, I think it's beautiful in it's own sad way (my english isn't THAT great, so this might also be a misunderstanding of some sort...). They took feelings away from Witchers that they won't feel fear when fighting monsters. Witchers have that code they follow, not their heart. They can't have children etc. They are like programmed robots. I know it isn't that simple, but I think you get the point. Yet Geralt feels so sad about Ciri, he kills the last Crone for revenge and that he can have back that medallion that was so important to Ciri. What could be more beautiful?