Hard to tell. Girls sports weren't really much of a thing as recently as 50 years ago. Now tons of girls are playing many different sports. So while girls may not be interested in flag football right now in huge numbers (and I'm just saying that for the sake of argument, there may be demand that I'm not aware of), I would guess it will grow like girls soccer, field hockey, volleyball, and lacrosse have. Even girls wrestling is growing in interest from girls. A local D3 college just announced they are adding a girls wrestling team. So there must be interest somewhere.Does anybody really believe that girls want to play flag football in any significant number? Maybe girls just want to be girls...why are we forcing these strange narratives on them that are largely appealing to young men/boys? Girls and boys are different, that is okay. FFS!!
I was in Florida last March with my son's HS baseball team (we do a spring break trip there every year) and stumbled onto a HS girls flag football game because the football stadium and baseball field shared the same parking lot. It was pretty interesting. Some of the girls were really good players and some were terrible. I think that is fairly common in girls sports though - much larger disparity of talent than in boys sports. I only watched a little over a quarter because my son's game was starting. But from what I observed, the girls were very engaged in the game and seemed to be loving it. And it had more fan support than I expected as I estimate that a couple hundred people were there.
I do find your remark that maybe girls just want to be girls and suggesting that someone is forcing them to do something to be a little strange. How would you know what girls want to do? Maybe, just maybe, the reason that they didn't used to play sports is because society deemed sports to be for boys and that it just wasn't offered. I don't have girls, but my boys have girl friends who play sports and love it. So why would you try to suggest it is wrong? There is still dance, band, choir, drama club, cheerleading, etc if the girls (or boys) prefer that to sports.
You whole premise is pretty insulting to women, IMO.