Quite simply, there are some cards that are just clearly broken.
Necropotence is one. It needs to go. It doesn't matter how you balance it out. It is just abusable. Seven or more extra cards at the price of three mana. It's broken beyond belief.
Now, Necropotence is broken, but, on it's own, it's damaging to play.
First, it seems to require mono black to achieve the mana. This is simply taken of with multi-lands, in most cases, and often Necropotence would be the heart of an aggressive mid-game strategy.
Second, it is vulnerable to enchantment destruction and countermagic.
Early Discard like Hymn can help here.
So, the traditional Necro-decks are good, but not broke as all hell.
But once too many cards entered the environment, it was bound to happen. Necro was abused. Easy life gain, 20 for four mana. That's good. But you lose it all in a turn or two. That's bad. So, there's a couple ways to do this...You could do some mad crazy Carnivorous Bloom or Stirge Familiar action, converting
life=cards=mana=drain life=cards=mana=drain life=win
That's probably the kind of deck I'd make out of this. But there was a simpler way. Donate the problem. Win next turn except versus Blue mana.
Now, the issue is, Necropotence is broken. The deck produces turn three kills. On average. Say they ban Necropotence. The combo is still there. Another drawing engine is required, but it could be done. I'm not sure a Illusion-Donate without Necro is gonna be competitive, but a controllish version, or a Replenish Version is possible.
So, another option : Ban Illusions of Grandeur. What happens now? Well, Illusions of Mediocrity does not a deck make. No upkeep means it's not an option. I'm not sure it would ever work, but if you really wanted, Donate-Forbidden Crypt is a combo deck. A version I constructed could average fifth turn kills. I'm sure a pro could improve on it. Although it really has a defiecency compared to Illusions, because twenty life gain is no joke. Generally, this is the best option in my opinion. Illusions of Grandeur was never played for itself. It was always Donate fodder.
The third option : Ban supporting cards. This could have been anything. Mana Vault, Ritual were the targets, but Demonic Consultation could have been a victim too. Now, the way I see it, Mana Vault was one of the most powerful cards ever, and should have gone the way of the Mox anyway. But Ritual, for a long time, in my eyes and I'm sure of others as well, WAS black. You simply did not play black without ritual. Broken ritual draws were part of the fun. I think that ritual IS too good. They shouldn't reprint it again. But they need to replace it with a strong replacement. A ritual with a compromise in some way. But the absence of a decent black in extended except for Trix, is the fault of Trix. The Trix archetype wasn't hurt enough, anyway. People still have to sideboard against it, and it is still quite strong. Combo decks, should, in my opinion, be the most dangerous decks to play. Difficult to survive beat-down, and easy for control to dissassemble, combo can only survive because of search cards. I think, to ban all search cards would end combo. Whether that is a good or bad thing, I don't know. But if Wizards hates combo, which I think is true, Wizards needs to get rid of searchers. Tutors, Consult, etc. More expensive searchers are actually okay. But it was the combination of speed mana and searchers that makes combo too powerful, and the actual combo is less relevant than the existence of combo support.
The question is : What is more damaging to an environment? The engine or the fuel? Necropotence or Ritual?