There are already plenty of crazy plot twists that make no sense in Diablo III.
If we had saved Leah in regular D3, it would have been another bullet point on a list of the self-appointed "hardcore" fans to whine about, but from a story perspective, based on the rest of the game, it felt like we should have and would have been able to.
It's such a hollow victory the way it is, and yet the people who actually want that "hollow victory" hated her in the first place and really don't have that feeling. I'd welcome a cheesy retcon that allows us to help. Because it feels wrong the way things ended, and I don't have a good way to fix it.
should have been able too? based on what? there's no story point that says the damsel in distress always needs to be saved. furthermore it presents a continuity error which is counter to what good story is supposed to have.
it felt wrong the way things ended? good. not every victory is without a price. diablo has been pretty hallmark on that where people get sacrificed to diablo and die with him.
i find it funny that the people who dont like her also don't want the story to change while people who do like her want the story to change and seem to have little knowledge of the previous games.
Honestly, it's not so much what happened that bothered me. It was the execution. You walk in, Leah's basically just moaning, Adria reveals her plan in explicit detail and then she's just gone. Then, nobody bothers to mention her in quest dialogue again.
I think it would have been better to see Adria's betrayal from the beginning, give Leah a last hurrah fighting Diablo from within to help us win (since it's flat-out stupid, IMO, that we take down the embodiment of pure evil with such ease), and say a few words for her when it's over. That alone would have been enough.
And as far as "should have been able to", I mean this: If her body is alive, if her soul is alive, if the soulstone is responsible for her status, all of which true in this game, it really doesn't make any sense that our characters wouldn't at least think about it. It was later discussed that the Diablo 1 Warrior had been fighting Diablo from within during the fight of Diablo 2, and while that was most likely a retcon it makes it sound as though he likely should have been able to have been saved as well, had we applied ourselves.
If you only want to talk about Diablo 2, though, go to the Diablo 2 forum. Diablo 3 has already clearly established that it is not interested in being held down by existing continuity. What was believed in Diablo 2 is not necessarily true in Diablo 3, and if you don't like it, you can go back to Diablo 2 because the original storytellers are not coming back.