They can in a papal election, why not in a vote on heresy?
Does that then constitute a Universal judgment by the Church? How Universal then if nearly half the Church disagrees?
Now, if these 49% were agreeing with Bergoglio that "Jesus is not God", then they too are obviously already outside the Church and their opinion doesn't matter. But if it's a disputed question about a proposition that's a couple of steps logically removed from dogma? What then? Does the 51% have the authority to oust them from the Church? What if the 51% are the ones who are heretical? Now, the Arians could easily have gathered a General Council in which they held majority and booted the non-Arians. What force would that have had?
Much of this is just theory, to help lay out principles, but the reality is much more ugly. Who has the authority to even convene a General Council if there's no universal consensus that one should even be held?