Theophylact of Ohrid

"And if the Lord had not shortened those days," that is, if He had not quickly ended the wars of the Romans, "no flesh would have been saved," not a single Jew would have remained. "But for the sake of the elect," that is, those from among the Jews who had believed and those who were to believe afterward, the war ended quickly. For God, foreseeing that after the captivity many of the Jews would come to believe, for this reason did not allow the Jewish people to perish utterly.

Commentary on Mark ↗

Bede

And unless the Lord had shortened the days, no flesh would have been saved. But for the sake of the elect whom He chose, He shortened the days. For this tribulation, as much as it is heavier in the weight of oppressions than the others which preceded, so much more will it be moderated by the brevity of time. For it is believed that for three and a half years, as can be conjectured from the prophecy of Daniel and the Revelation of St. John, it will wage war against the Church throughout the world.

On the Gospel of Mark ↗