@snowydude Mana drain is not played in gush decks.
Sudden shock is not a good card. If LSV would have played properly and held priority to gush in response to his mox pearl to generate a second token, Efro would have lost the game despite having his "perfect" answer to mentor. If you know or suspect your opponent to have sudden shock it's incredibly easy to play around and isn't much different from a 2 mana shock. Playing 2 mana shock against an archetype that can draw more cards than any other is not a winning line. Also having your removal be 2 mana shocks against thorn decks loses you huge percentage points in those matchups.
Dack isn't particularly good in a blue mirror without gush. Dack is insane when you are short on cards but can gush excess lands into your hand and discard those. Without gush, JTMS is hard to cast, and even harder to resolve or protect because you aren't up a bunch of cards on your opponent. Mentor is a stupid card that isn't fun to lose to, but that doesn't make it better than pyromancer. I think you are conflating a card being the one that wins the game too much with being the card that makes the deck work.