Rav Adda and Rav Shalmon give a diff. reason why the egg on the 2nd day of RH would still be prohibited nowadays. The Bais Hamikdash will come and the Takanah of 2 days would automatically resume (and someone might use an egg on the 2nd day).
If we say that the Takanah is only removed with a gathering of Rabbis, not just because the reason went away, why would a Takaknah go back in place without that same gathering (and if there was a gathering wouldn't there be a "Kol?)?
Thanks for your question. In the Mishnah in Ma'aser Sheni (5:2) it mentions that according to Rabbi Yosi, the requirement to bring the actual fruit to Yerushalayim was negated when the Beis ha'Mikdash was destroyed. Rabbi Yosi concludes that at the time of the Churban, they stipulated that when the Beis ha'Mikdash will be rebuilt, the original requirement will automatically be restored.
The Tiferes Yisrael explains that this stipulation was necessary, for otherwise they could have not annulled the original Takanah. This shows us that when Takanos were annulled as a result of the Churban, it was done on a temporary basis until the Beis ha'Mikdash will be rebuilt. I would assume that this idea applies here as well. When the Beis ha'Mikdash will be rebuilt, bi'Meheirah b'Yamainu, the orginal Takanos will be restored automatically.
May we speedily be zocheh to see the Beis ha'Mikdash rebuilt.