The title statement is the Gemara’s justification for why Diaspora communities continue to observe 2 days of Yom Tov, despite certain knowledge of the date, because of the fundamental uncertainty of existing in Exile.
There’s a great passage in Nachum Glatzer’s intellectual biography of Franz Rosenzweig (and I can’t find the passage) where he borrows the statement “the tradition of the forefathers in our hands”, to refer to the preservationist tendencies of exilic communities.
I’m not sure if this is what he intended, but it’s a great point: the 2-day yom tov is a paradigm for ‘galus’ halakhic approaches in general; an insistence upon maintaining outdated elements of a tradition, a desire to link with and preserve the past, an overshift toward ‘elokei Avi ve-aromemenhu’ at the expense of ‘zeh Keli ve-anvehu’. It’s really a beautiful idea.
Writings of R’ Kook and Eliezer Berkovitz (and, I now know, the R’ Y. Y. Weinberg thought this way as well; see the last chapter of Marc Shapiro’s biography) are full of contrasts between the nature and role of the Halakhic system in Exile and under Sovereign conditions (like in the modern State of Israel). Dr. Berkovitz (following his Rebbi) makes a much more specific, practical, and polemical argument (chapter 4 of Not In Heaven).
Though I’m not so familiar with their works, R’ Chaim Hirschenson and R’ Shlomo Goren apparently very much thought along these lines as well.
It’s implications for the possibility of a Halakhic renewal in Israel are profound and exciting to think (perchance, to dream) about.
Unfortunately, it seems that the Galuti approach to Halakha yet prevails. Perhaps someday we’ll emancipate ourselves from it. In the meantime, I’d highly recommend those two chapters (in Berkovits and Shapiro) for a fuller picture, and I’ll try to find that reference in Glatzer.