The best solution I’ve heard started by looking at who benefits from this norm [older women] and wondering whether they could have contributed to it. After all, the strength of this norm has been increased in the last sixty years or so, which coincides with the period in which women’s power has increased.

The primary mistake you're making here is confusing correlation and causation. Even if you assume that the feminism movement has been the sole cause of this, the status gains older women have made are orders of magnitude smaller than the status gains all women have made. (If you disagree with this, I'm going to tap out and let someone who is more well versed in the history of feminism make this argument.) This may have been a small bonus for them (I doubt it even entered anyone's consideration) but to me this seems like saying people invest their money because they don't want to store it in a chest in their closet. Sure, it's nice that they don't have to lock it up, but the primary reason they are investing is to make more money.