Perhaps a better technique would be to go around the room and require everyone to come up with the best argument for why the plan isn't sane, regardless of whet

Perhaps a better technique would be to go around the room and require everyone to come up with the best argument for why the plan isn't sane, regardless of whether they actually believe their argument. That way the argument isn't attached to anyone's ego, and uncomfortable truths can be spoken with plausible deniability. It only takes one person to "jokingly" bring up how previous estimates were over-optimistic, for example, to which other people can nod and hum in agreement.