In reply to your question as to whether a rabbinic decree could be revoked, there is a concept in Jewish law that prescribes that once a rabbinic decree is acce

In reply to your question as to whether a rabbinic decree could be revoked, there is a concept in Jewish law that prescribes that once a rabbinic decree is accepted by the Jewish nation (I.e. Widely practiced) it becomes law. So even if you would maintain the two hour buffer is no longer necessary, it would remain as law in order to uphold tradition. As an example, the rabbis decreed that Jewish holidays outside the land of Israel should be celebrated for a second day. The reason being that the court which established the new month (based on witnesses who had seen the new moon) was based in Jerusalem, and the Jews outside the land of Israel would not know when to celebrate the holiday, so out of doubt they celebrated the day after as well (in case the month was a day longer)[0]. When the time came that the court foresaw that they would soon no longer be able to establish the new month (due to the exile of the Jews from the land of Israel) they established a calendar by which the new month was to be determined. At this point, the Jews in the land of Israel and outside had the same calendar, and knew exactly when to celebrate the holidays. Nevertheless, the rabbs maintained a distinction between inside the land of Israel, and outside, where holidays are celebrated for a second day until this day. [0] A Jewish month can be either 29 or 30 days