I was at the AGM, where Subbies position on the matter was outlined.
From my recollection of the Chairmans message the below was noted:
Player numbers, especially towards the end of the season were dwindling, and many 3rd graders were playing two games to make 4th grade happen.
1st Grade were not at the level required to compete in the division.
Analysis was completed around the strength of each win vs the losses. IE Each win they achieved was within X points compared to the losses which gave an average of Y points (or something to that effect).
The decision was handed down with the "finest of margins", but ultimately Mosman was deemed to have had a 1 off bad year across the grades, but still had numbers to supplement them staying in the top tier.
Subbies also mentioned that their promotion may have come pre-maturely when they were asked to come up. Subbies fell on their sword on this point. A bit like 'its not you its me'.
Dont shoot the messenger. But this was my best recollection of their main points of Defence. I cant recall the exact numbers of the votes, but lets just say it wasn't close.