R. Yochanan had answered a hard problem brought up by his teacher and his teacher said "This is no ordinary human being." At that point the Gemara brings up this idea that "If previous generations were like angels, we are like men etc."
However the Rambam disagrees with this idea. He sees no problem in disagreeing with the Geonim [Example in Hilchot Ribit], or the Rif [e.g in Ketubot] [see in the laws of Mishpatim and the beginning of Mishna Torah where he discuss the rules of deciding Halacha]
Also this idea is clearly in contradiction to the other idea mentioned by the Gemara itself that the law goes by the last Amora [the later Talmudic sage]
Now sometimes the idea of the falling of the generations makes sense. I encounter this all the time when I see the difference between Tosphot and Achronim [later commentaries] that come after the Beit Yoseph.