I suspect economics played a very large part, with regards to the inclusion of non boys.
I agree with most things you say but in this regard, I think it's more along what
Ogertron3000
said, political. I'll try to skirt the subject to avoid a political debate or discussion as per our TOS but I think the motivation is the same as it's been in all similar moves in sports such as weightlifting, swimming, etc which is all-inclusiveness. My opinion on the motivation behind the all-inclusiveness I'll digress on.
I too was a boycout and enjoyed the experience matter of fact my Mom was a Co-Troop Girl Scout Leader with her friend for my sister. I think it was a positive experience as the motivation to get badges, recognition, cooperation and respect shaped a lot of my most positive personality traits. For instance, my Father believed in a do just enough to get by when it comes to work so as not to become someone that's asked more of, but I couldn't help myself my Mother and the boyscouts had instilled in me to always do my best. One could argue that he was right as I was given many paper certificate awards but was passed over for promotions that would improve my pay if it didn't also garner more for the company. So, I was always getting more responsibility with no monetary gains until I was laid off when I broke my back which took all of a week for me to get the axe. Still, I take some solace in that to this day and while it is often a struggle to not compromise my own personal morals and ideals. To respect myself even if often times the world won't.
Forgive the large amount of text, I've always been a motor mouth and please forgive the whiny self-righteousness as I get older I complain more and perhaps I'm getting an over inflated sense of self.