Peter: "Off the top of my head... "plundered by robbers and thieves" seems to imply a threat from outside and in that list that seems to only match with "refute adverse opinions". If that is so, the it means admitting women to the order makes the order as a whole less able to refute adverse opinions. I don't know why that would be, but there it is."
My understanding that admitting women into the order may weaken the order may due to the following reasons.
1) The nature of human that naturally gravitate towards the notion of opposite attracts. i.e. our nature is to reproduce or to mate just like animal.
2) The weakening of the ability to destroy the hardest of the Three Unwholesome Roots namely desire or craving. Again relating to the notion of opposite attracts - desire.
3) As the time passes the Unwholesome Root of desire started to slowly chip away the core of Buddha's teaching namely destroying the Three Unwholesome Roots. Rules may become lax. If I am not mistaken Desire is the hardest to eliminate.
4) The stronger the desire in human the less they are able to sustain and to practice the strict Buddha teaching.
5) When the strict teaching of Buddha is no longer observed there will be breaches and this also means the teaching of Buddha will become prone to criticisim. Scandal etc. i.e. external attacks / criticisims may be targeted at the perceived weaker sex to the point that they can no longer be contained.
6) When scandals being to take effect the confidence of the people in those who continue to teach Dhamma as taught by Buddha diminishes. i.e. if the "teachers" cannot follow the proper codes of conduct how can they teach others, as the teachers have not reached the stage where they themselves experience Dhamma in the purest sense.
7) With diminishing confidence of the teachers teaching Dhamma the Sangha slowly disappear and the end result is inevitable.
8) My guess is that to exclude women to the order is merely to help guard our senses against desire of men and women.
9) Another point is that women is traditional regarded as "property" of the men and even in our modern society women is still regarded less than men. i.e. a weaker sex in general physical sense.
10) Therefore, it is not because women cannot be arahants, they can (many attained it during Buddha's time), but merely that having women in the order strengthen the Three Unwholesome Roots of desire that leads to other things and reduce the effectiveness or the will of those that follow Buddha's teaching strictly in a longer run, which also leads to the less people attaining arahantship due to impurity (distraction) by comparison to say if there is only one sex (one problem less in the realm of desire).