Rabbi Avraham Dov Ber Kahana Shapiro of Kovna, author of “Devar Avraham”, once explained the decreasing order in Avraham’s prayer for Sedom (fifty good people, 45, 40, thirty, etc.), saying Sedom was just like Kovna, i.e., in a precipitous decline.
So, as time passed, Avraham lowered his estimate of righteous people…
(By the way, Rabbi Shapiro was also part of the “Netzach Yisrael” secret society in Volozhin.)