@p3temangus Drawing cards is great and Dack and Jace are great at drawing cards. What are you even trying to draw with Dack and Jace?
Jace and Dack are ok at being answers and ok at being threats, but they are really good at drawing cards. Chandra and Nahiri are good at being answers and great at being threats and only ok at drawing cards. Being able to kill creatures is a pretty useful ability. They can answer a Leovold where Jace and Dack are pretty useless. They can kill a Thought-Knot Seer or a Thalia or a Dragonlord Ojutai. Nahiri can kill an Oath of Druids. Chandra can kill a Jace. Chandra and Nahiri are just better against the things people play to beat the rest of your deck.
Chandra and Nahiri are more resilient. They have higher loyalty, better defensive abilities, and quicker ultimates. Not say Jace isn't a fantastic win condition. He is. It also sucks to have your four mana investment Pyroblasted. If that sounds like "Dies to Doom Blade" you're not wrong. The way to win Vintage control mirrors is by blanking all the Doom Blades. Making their Swords, Pushes, Blasts, Verdicts, and Decays dead is worth giving up the fourth Mentor or Dack or Jace.