Q. Why do we celebrate two days of yom tov in chutz la’aretz (and rosh hashana even in Israel) if we now have a set calendar?

A. This is an actual question of the Talmud (Beitzah 4b) “But now that we are well acquainted with the fixing [by the calendar] of the new moon, why do we observe two days? — Because they sent [word] from Eretz Yisroel; Be vigilant on the customs of your ancestors which have come down to you; for it might happen that a government might issue a decree [forbidding Torah learning and we will forget the establishing of the months – Rashi] and it will cause ruin [to our traditions], see also Mishna Berura 496: 1

Meshech Hochmo (Pashas Bo) writes that in the near and close future the Great Sanhedrin of Yerushalaim will be re-established and they will again institute kiddush hachodesh by the testimony of witnesses. If then the new moon was not yet seen by the thirtieth day, even though according to the set calculated calendar it is already Rosh Chodesh, they will postpone it until next day. Therefore even today, we keep two days for the sake of a hopefully prompt new future.

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit”a