I think you hit the nail on the head, and I actually wrote another story a while back on this topic where I talked about calling black men "boy" as a way to indicate that they didn't have the same social power or importance as white men - and how this is the same kind of dynamic in persisting in calling women girl. I didn't go into this, but the same goes for Uncle as in Uncle Ben, which exempts white people from having to call a black man Mr. even though he is an elder.
I do think that one of the reasons so many women still call each other girl is the aspect you've named where culturally, it is better for a woman to be young. That's what's seen as sexy, and since that's a woman's main cache, many subconsciously don't want to give that up, which is why I asked those who feel uncomfortable with the word woman to think about why that may be so.
"Another aspect of this issue is that adult women are culturally not seen to be as valuable as young, nubile, fresh young girls. We don’t have to take them seriously, but damn… they look good! Some women want to remain as girls for this reason. There really is no term for females other than girl that conveys vibrancy, beauty, sex appeal, and desirability. Some women resist being labeled as such because it sounds old and dried up to them. Broads, chicks, dames…. there really is no other good word for women that conveys beauty and power other than girl (such as her power is — simply to be hot and sexy)."
