The Girl Scout Organization is the one with the most lose in this. Girls that want the outside camping and high adventure programs that Scouting offers are going to make the jump. Some others will be looking for the lifelong honor bestowed by the rank of Eagle. And for teens, hey it's co-ed camping!
Not surprisingly, the president of Girl Scouts of the USA called the Boy Scouts’ “covert campaign” to recruit girls “reckless” and “unsettling”. It might be reckless, but it's only unsettling to the Girl Scouts at this point and it will be anything but covert. Scouting is going to be recruiting hard, trying to build it's numbers back up, and recreate itself as still a meaningful organization for young people in this century.
Scouting is a corporation. It makes money, pays salaries to it's professionals, owns a great deal of property. Scouting sells the patches and uniforms, owns the Council summer camps, provides the training materials, etc. This provides a framework for thousands of volunteer leaders to dedicate millions of hours every year to run Troops.
I'm not saying this is bad or good, that's just how it worked out. When I was a Scouter, I ignored as much of that as I could and focused on running a great Troop, developing the Scouts as leaders, running an active camping program, and giving the every Scout the opportunity to advance up the ranks. If I was still Scouting, I would do the same thing again.
Girls won't hurt the program, but they will make the logistics of running the Troop more complicated. You will need female leaders willing to participate in all activities. Summer camps will need new shower houses and bathroom facilities. Guidelines will have be established on how patrols set up campsites. That list will go on and on. It will take a decade or more for the change to be fully implemented and then we can all assess how it worked out.