I believe that Cain is the one who narrates the destruction of the soulstones as the destruction of the demons, or insinuates it- that they would be sent back to Hell, proper, I think?
While Cain seems to be the Oracle of the Omniscient Storytellers, he is only a character in the objective (fictional) universe. While he is presented infallible, he is, in fact, fallible, as are any and all writers/speakers in the series. Only the reality of the cosmos is infallible.
The human body can host the soul of the prime evils, c.f., Tal Rasha and the Dark Wanderer. While they had shards stuck in the heads, I don't think that precludes the possibility that a human vessel, without assistance/assisting, could house the souls/spirits of the Evils.
Also, keeping in mind, the souls of the Prime Evils had to be ethereal, even in the beginning, to transfer into the original soulstones. Effectively, what has happened, since the Black Soulstone existed, they just got... sucked into it, like a vortex, after the destruction of their soulstones.
Now, at the end of Diablo 3, what seems to be unclear, is what will happen now, as the soul stone seems to, as yes, be in tact. What if it burns in re-entry, breaks apart, and the Seven are reconstituted for some final fights, plus the mini-bosses, for one last go around.... What if new cults and orders are set-up, under the nose of Zakarum and the heroes of Sanctuary?
The expansion(s) could be longer than the original/core game!
Edited by Ronin#1195 on 1/3/2013 8:33 PM PST