Imho that is NOT the diamonds gates that Tyrael will smash but The CRYSTAL ARCH himself, which has been corrupted by diablo. Thus the power source of Heaven will be gone and no more angels will be reborn from the Spire of Anu (which is the crystal arch).
Like that Diablo III will have the exact epic ending as in Diablo II Lord of Destruction. Hell will be wiped out (we killed them all at the end of Diablo III, maybe Belial is still out there but he is no big deal when facing himself). Heaven will be erased leaving only a few survivors like Tyrael alive.
See you in the expansion pack fighting the fallen angel (i am not refering to Tyrael)