That's like me observing the greater numbers of western Pennsylvanian teens who hike and camp with their mates independent of BSA or GS/USA and betting BSA lifting it's ten-year-old ban on patrol overnights. Not gonna happen.
Venturing does a turnaround and becomes overwhelmingly popular, or
Major political upheaval and an opinion from POTUS, or
Market surveys show large segments of young males who will only participate in co-ed organizations, or
Boy Scouts start demanding that BSA open to girls.
Although I believe the last scenario the most likely after the next world jamboree, I don't see any of those scenarios making boots-on-the-ground leaders happy. We would likely follow the Brits' trajectory and loose many units until a new generation of leaders could be trained to fill their place.
I agree wit some and disagree with other points here.
I certainly agree that hosting the World Jamboree in 2019 will be highly influential. There will be an awful lot of both scouts and adults who will be exposed to coed scouting for the first time. Those who camp at the jamboree, the many more who will go on day trips, families who host others as part of home hospitality. And their friends. I would anticipate a greater acceptance of coed scouting following that.
If memory serves, and Ian may correct me if I am wrong, the admission of girls to all sections here followed a number of groups which just started unilaterally accepting girls. It was a bottom up movement among certain groups and HQ eventually went along with it. Certainly that was the impression I had, bare in mind I was only 13 so perfectly capable of getting the wrong end of the stick!
I think you may be over egging the pudding in terms of linking the drop in numbers in the UK with going coed. There was an element to it I would agree, a number of adult leaders who quit as they didn't like the way it was going. However, being a scout and Venture scout in that period I wasn't aware of a single youth member that quit because of it. The issue was more the program, the image and the terrible uniform for the many of my friends that quit while I kept going. That's not to say that you wouldn't have initial losses. Any organisation that goes through a fundamental change will suffer losses, either in protest at the change or before the change if it comes too late. Either way there would be fall out. Here in Cambridge a number of the university colleges saw academic staff resign in protest as one by one they were opened to women. Such is life.
If I were BSA HQ at the moment I would be steering clear. The impression I have is there are an awful lot wounds being licked following the settlement of the gay issue and making a fundamental change to the membership model is probably the last thing they want to get involved right now. Equally though that very issue, not wanting another fight, could result in a blind eye being turned to units having girls as unofficial members. I suspect that, one day, the change will be made. Across the world previously male only institutions of all sorts have only gone one way.