Vile taint is less of a problem than shadow crash because Affliction’s AoE isn’t entirely reliant upon it. Shadow being capped at 8 targets becomes a large issue whenever there’s more things to hit than that.
You can’t tie agony to seed of corruption without having the same limits that vile taint already has, or else you’re going to be looking at nerfs to Agony or MR to adjust the sudden increase of how many targets you can easily slap Agony on. Vile taint already does the job it needs to do by limiting Agony without needing to limit Wither or Corruption application.
Aff’s biggest issue, especially in fights with small adds that do not live long, is how sluggish seed feels. It’s already a long cast, and then it takes further time to explode on fresh targets. It just isn’t fast enough to work in quick aoe situations.