But if you say you want to pull big everytime at the start of the dungeon then scanario b) will not happen and scenario a) is so incredibly rare with our current dungeon structure that it's also unlikely to happen that they don't see the next trash pull.
The thing that makes rescuing tanks into the next trash toxic is the fact that you take away control from a player who can decide for themselves if they want to pull more or not. It's /their/ role to take aggro, no one should be able to physically force that against their will. That's literally the only issue I have.