How many SOs do we have now? Getting on for 500 I believe. Only a handful have not joined the AOA- through laziness more than anything. You can be sure they will all leave the AOA if the union does not fight some of the leapfrogging scenarios being mentioned.
You may say they don't yet have the minimum requirements in terms of hours to hold these FO positions etc. Trust me the safeguarding of these positions etc due to seniority must still be fought for.