What's keeping someone from running Ysera, Alexstrasza, Ragnaros, Sylvanas, Boom, Tirion, Kel'Thuzad and Sneed's Shredder in a single deck? All of those are solid cards, most are played regularly in some decks, albeit not all in a single deck.
Answer: a deck like that would have too high a chance of having a starting hand full of 6/7/8 drops and would suffer too high a chance of being blown up by face hunter or ebolapaladin before they even play a card. If you get rid of face hunter, then there is no longer any clear penalty for just playing a priest who heals himself until turn 5 and then drops a sludge belcher and then drops giant bomb minions every turn after that.
You want to find a middle point where all of these deck arechetypes are viable and none begins to overshadow the other archetypes. Right now, aggro is probably a wee too strong, but you might be just as annoyed if it swung in the other direction and now mill decks and super bloated control decks took over instead.