Commentary for Kiddushin 131:12

A much later amora interpolates into the story that Yannai’s response is heretical for it assumes that the Torah is limited to the Written Torah. The Oral Torah requires the sages, and if Yannai destroys them, it will be lost.

The story’s end seems to be, at least partially, an explanation as to why we know of so few sages from an early period. After all, if the Oral Torah was given on Sinai, then we should have rabbis from a much earlier period. One explanation, the one given in our story, is that there was a break in the transmission of Oral Torah, and that all of the sages were massacred. Shimon b. Shetach restored the Torah to where it was, and thus our Torah is legitimate. But still, there was a break.