Quote fromHis soulstone was shattered prior to Diablo 1. Tal'Rasha had one shard and used it to contain Baal for a long long time, but eventually lost the battle.
So, while it is shattered, it is not completely destroyed.
Which brings up the question: When you smashed Diablo's and Mephisto's soulstones, did you just shatter them, or did you completely obliterate them?
As for the use of the Soulstones, they were used to imprison The Three centuries(?) ago, by Cain's ancestors I believe.
Diablo's soulstone was also shattered in a similar manner to Baal's; the soulstone which was eventually embedded in the Wanderer's head was technically just a splinter of it.
It was stated fairly clearly that, by smashing a soulstone on the Hellforge,
you banish the soul within from the world. The destruction of the Worldstone seems to be the only loophole around this.
Back on the topic post:
At the end of LoD, it simply would've been anticlimactic to see your character or Tyrael trek all the way to Hell just to smash the stone, and it would basically confirm that the world survived (which we didn't necessarily know yet) or that Tyrael was still the same being.
The Diablo III writers, who may be working on prior frameworks left by Blizzard North or may be creating the story from scratch, can interpret this either way.