It was actually the subject of a machlokes Rishonim if Rosh HaShana should be two days in Jerusalem/Beis HaVaad/Israel. The Baal Me'or (Beitza 3a) cites part of the discussion, and records that the practice was to only keep one day, until it was changed per the psak of the Rif.
The Rif claims that based on Rava (that R' Yochanan would agree even after the enactment of Raban Yochanan Ben Zakai to return acceptance of testimony to the whole day, that an egg born on day 1 of Rosh Hashana is still forbidden on day 2) that even in Israel Rosh Hashana should be 2 days.
Rabbeinu Efraim (cited by Rosh and Baal Me'or) says that once the calendar was established, all of Israel is like the Beis HaVaad where there is no doubt and they keep one day.
The Rashba and the Rosh defend the Rif. The Rashba says that from the fact that outside of Israel didn't change with the establishment of the calendar, we see that the establishment of the calendar didn't change these things. The Rosh says a slightly different variation - that from the fact that outside of Israel they still forbid an egg born on day 1 of Rosh Hashana to be eaten on day 2 of Rosh Hashana, we see that even after the establishment of the calendar it is still treated as one long day.
The Ramban says a different proof - that all that needs to be proven is if the 2 days of Rosh HaShana were an enactment or just a reaction to a doubt. Once it is proven that they are an enactment, they do not revert just because the doubt is resolved.