You need a community with a steady population of 18 to 21-year old men and women who are interested in the outdoors, or else the program has no leadership. In most towns, Venturing programs crumble because youth leave the program when they graduate from high school. Where it has been successful, the young women seem to have taken the strongest roles in Venturing leadership. At least two of the years I was at the National Meeting, it seems that three of the four Regional Presidents for Venturing were young women.
Over the last five years as a Scouter, as I've become more involved in International Scouting and seen that all but two of the 140+ WOS Scouting programs have successfully integrated boys and girls, the more inclined I am to continue to fight the God and gays battle, and to view favorably the possibility of a co-ed Scouting program at all ages.
The problem in the U.S. is that Girl Scouting is so well-developed, and so different than Boy Scouting in many ways, that I can never see the two programs merging. We'd simply have to call our program "Scouting," let the girls in, and let the Girl Scouts fend for themselves.
I know the idea of girls in Scouting is taboo for many older Scouters, but I've seen the successes of the Venturing program, and the successes of International Scouting programs that are co-ed.
The GSUSA and the BSA are highly unlikely to merge. Work has been done on it, but their methods are too different. After what I've seen in from other countries, I think that you're right; that the BSA could open up it's membership to both sexes at all age levels and make it work. But it might be wise to at least leave the option that Boy Scout Troops could be single gender (of either gender) should the sponsoring organization so choose.
My Council (Des Plaines Valley Council) has a small but very active Venturing program and I've been very impressed with what I've seen. And yes, young ladies tend to gravitate into the leadership positions.
BTW, short backgrounder: Cub Scout (Bobcat --> AOL), Boy Scout (Life), Camp staff 2 years, High Adventure at Matagamon Canoe Base, OA; left the program at age 17 when I came back from college with hair down to my the base of my neck and the SM and CC took me aside and told me I looked like a pimp. Adult: CM 4.5 years, SM 7.5 years, did my | >>>------> |, staffed various camporees, staffed various training courses, ran a couple of training courses, got my beads from C-19-96 (Eagles), C-30-02 (Staffer), currently SPL for C-19-06, got DAMmed, CM award, Scouter's Training Award, Scoutmaster's Key. I've run the District Klondike Camporee for the last 5 years, but it's someone else's turn now. When I stepped down after my son got his Eagle and turned 18 from SM to ASM, the district made me District Commissioner. MC of our Pack, ASM of our Troop, AA of the Council's leadership Crew and DC are my current assignments.