Two questions related to uprooting a previous gezerah
1. Shulchan Arukh based on Tosafot and other Rishonim state that now
women wear jewelry in public. One of the reasons is that a reshut
harabim does not exist today. Another reason is that women today
no longer take off jewelry to show their friends since jewelry is
now more common and worn all week long.
2. Rabbi Akiva allowed previously forbidden activities so women would
appear attractive to their husbands and so not cause divorces.
We have a general rule that one cannot disagree with a previous gezerah
unless the new bet din is greater than the first even if the reasons
no longer apply.
Why are these two cases exceptions to the rule?
(a) This is a far-reaching and important question. TOSFOS (Avodah Zarah 35a), concerning the prohibition of drinking water that was left uncovered (which was enacted due to the fear that a snake might have emitted venom into the water), writes that the Rabanan enacted the decree "only in places where snakes are commonly found," and therefore the prohibition does not apply today in most civilized places. The same type of reasoning can be applied here, according to the opinion that women may go out with jewelry in places where there is no Reshus ha'Rabim; the Rabanan did not enact their decree except in places that have a real Reshus ha'Rabim.
According to the opinion that says that women today do not take off their jewelry to show to their friends, either because the women today are considered noble and it is below their dignity to remove their jewelry to show others, or because they are accustomed to wearing jewelry and there is no novelty in wearing jewelry, the question is more difficult to answer. It must be that the Rabanan enacted the decree only for women who are not considered noble and who might remove their jewelry to show to friends. The issue, though, still requires further elucidation (see TOSFOS in Avodah Zarah 57b, DH la'Afukei, who suggests a similar approach regarding the ezeirah of Yayin Nesech).
It seems that TOSFOS (64b, DH Rebbi, according to Rabeinu Shimshon) differentiates in another way, for they say that the Rabanan enacted the decree only on jewelry which is normally removed to show to others, according to the customs of the times.
(b) This is not related to the issue of uprooting previous Gezeiros. In this case, the Rabanan prohibited a Nidah from wearing certain types of make-up derive, basing their enactment on a verse (as Rashi explains here, DH b'Nidasa). Rebbi Akiva, however, uses the verse for other teachings, and maintains that the Rabanan made a mistake, and therefore he permits a Nidah to wear those types of make-up.