You don't know whether you have consensus before you discuss something, and sometimes discussion modifies the consensus.
Discussion is the only hope of evaluating that.
The walls around your mind come tumbling down, and the world is a cold and drafty place. You have to test each step, picking your way through the rubble. Having started questioning everything, you're going to find that some things are true, after all. The debate about senority established that most people contributing to the discussion, having considered the matter, feel it is a legitimate, reasonably fair method, applicable to this particular case. There are so many issues that get brought up that nobody ever replies to.