PAA,
Nishtanah HaTevah is something that is used to describe clear physical changes from the times of Chazal until now (like vest kavuah or the size of olives/fingers). The problem with minds is that we can’t know what the tevah was in the past, or if any change that has occurred is sufficient to necessitate a change in education policy.
That being said, it is clear from the Rambam and Jewish history, that even in the past there were women of exceptional Torah scholarship who learned kol haTorah kula. Rabbi Eliezer’s opinion should therefore be viewed as a general rule, but for the exceptional woman who is highly motivated, she can and should learn (as an ainah metzuva v’osah). Query whether the percentage of exceptional women is higher today than it was in the past.