Unfortunately this was always going to occur either way when you consider the foundations Melbourne built it's troop on (all older, hand raised females).
In regards to the future, I can only really see one option here if they're unwilling to take the risk adding any other additional females;
That is maintaining the group as it is for the foreseeable. Maybe with the intention of acquiring an entirely new troop in the long term and moving this trio across to Werribee once their troop starts to pass on.
*Maybe by that point, we may see the remaining male given the opportunity to head up a troop at Melbourne(wishful thinking!)
You make a good point regarding the challenges Melbourne had in establishing a functional troop considering Yuska and Betsy were hand-raised (albeit with peers); and then as if that wasn’t enough, they had an infertile silverback and another that was socially malapted to lead the troop. Fast forward to the import of Otana, who has been far from ideal
Compare this to Taronga, who imported a mother raised family troop from Europe; with a equally impressive male heading up the current troop.