Dredge can't dominate the metagame because if your opponent happens to throw four Crypts in their board you suddenly have a bad matchup.

Combo's dominance has been theorized but I don't see the logic behind it. They also use Brainstorm pretty heavily, they're not at all widely played right now so combo decks becoming maybe 20% of the meta isn't that terrifying, and they don't become any less fragile by banning Brainstorm; rather, it generally has a negative impact on their consistency.