I think many of the disagreements about the narrative dissonance of ret-conning come from the perspective that a player's past actions dictate agency. This is, of course, something one can agree with. There's also the perspective that making a decision in the moment defines agency, and that ret-conning which is subsequent does not render null any agency which the player may have once had. However, in real life, it seems fairly reasonable to say, pragmatically, that you don't have agency in a world where what you did in the past has no relation to the action which you took in the past, et cetera.
As for the argument that a game mechanic can't drive a story, I'm inclined to agree that the point of a role-playing game is to allow the player to play their role, not to make the player a vessel for chance, but, as prabe mentioned, a game mechanic does not inherently violate the conventions of the simulation. I'm likely rambling, as I always do, so, I'll try to make the point a bit more concise.