There is a debate about decrees from the sages if once the reason for the decree has gone, if the decree itself is automatically null. This is an argument in the Gemara itself [Beiza page 5]. Tosphot takes it as a simple fact that the decree itself is null and so does the Raavad and most other rishonim.
The reason this is relevant is that for most decrees we know the reason for (as they are stated openly in the Gemara itself). In any case, the Rambam disagrees and hold the decree continues until another court of law can be convened to nullify the decree. However that is not the opinion of most Rishonim.
In fact, there is even a debate if there is any authority in the first place to make a decree which is not stated openly in the Torah, the Law of Moses. This come up in the beginning of אבות דר' נתן which is a commentary on Tractate Avot by an Amora printed in the Vilna Shas at the end of Nezikim.
Besides that there is also the issue of ר' שמעון דורש טעמה דקרא {Shimon ben Yohai goes by the reason for the verse, not the letter of the law. This is actually how the Rambam decided in one place in Mishne Torah. In another place the Rambam decided like the sages that go by the letter of the law. The commentaries on the Rambam are at a loss how to deal with that, however the Avi Ezri answers the question quite well.] In any case I want to point out that Shimon ben Yohai is quite serious about going for the reason for a verse as you can see in the case of a wealthy widow. He says a lender can take a pledge from her even though the verse openly says one can not do so. The reason is that he goes by the reason for the verse, not the letter of the law.
In fact, there is even a debate if there is any authority in the first place to make a decree which is not stated openly in the Torah, the Law of Moses. This come up in the beginning of אבות דר' נתן which is a commentary on Tractate Avot by an Amora printed in the Vilna Shas at the end of Nezikim.
Besides that there is also the issue of ר' שמעון דורש טעמה דקרא {Shimon ben Yohai goes by the reason for the verse, not the letter of the law. This is actually how the Rambam decided in one place in Mishne Torah. In another place the Rambam decided like the sages that go by the letter of the law. The commentaries on the Rambam are at a loss how to deal with that, however the Avi Ezri answers the question quite well.] In any case I want to point out that Shimon ben Yohai is quite serious about going for the reason for a verse as you can see in the case of a wealthy widow. He says a lender can take a pledge from her even though the verse openly says one can not do so. The reason is that he goes by the reason for the verse, not the letter of the law.
