Ten generations from Adam until Noach. To apprise us how long-suffering He is. For all of these generations they kept on angering Him — until He brought upon them the waters of the flood. [You, too, do not wonder that He has been long-suffering to the idolators all of these years that they have been subjugating His children, for He was even more so to the idolators of the (ten) generations from Adam until Noach — until they were destroyed in the flood.] Ten generations from Noach until Abraham. To apprise us how long-suffering He is. For all of these generations they kept on angering Him — until Abraham came and took upon himself the reward of all. [He did good deeds corresponding to what all of them should have done — wherefore they were all rescued in his merit. And just as he took upon himself the yoke of mitzvoth in this world over and against all of them, so did he receive for the world to come reward over and against all of them. For every man has two shares, one in Gan Eden; the other, in Gehinnom. If he is meritorious, he takes his share and that of his neighbor (the evildoer) in Gan Eden. If he is culpable, he takes his share and that of his neighbor (the tzaddik) in Gehinnom.]
Kli Yakar
Another interpretation of “Remember the days of old” follows the approach of our Sages of blessed memory (Avot 5:1): “The world was created with 10 utterances. What does this teach us? Could it not have been created with a single utterance? Rather, it is to exact punishment from the wicked who destroy the world that was created with 10 utterances.” This means we say to the wicked person: “See how evil you are, for you are destroying something great, as it was created with 10 utterances.” Similarly, regarding the six days of creation — for the same reason — God could have created it in a single moment, but this teaches the wicked person that through his sin he destroys something great that was created in six days and with 10 utterances. The Holy One, blessed be He, established foundations for the world in the Written Torah, which is encompassed in the Ten Commandments, and in the Oral Torah, which is encompassed in the six Orders, indicating that the world’s existence depends on them. Therefore, He said, Remember the days of old — that the world was created in six days — and guard yourself from destroying this great thing. Similarly, consider the years of generation after generation — these are the 10 generations from Adam to Noah, and from Noah to Abraham, as it is said: Though a sinner does evil a hundred times and his punishment is delayed (Ecclesiastes 8:12). You might think that since the Holy One, blessed be He, is slow to anger and does not collect His due immediately, there is no justice and no Judge, and therefore your heart may be filled to do evil. But you would be trusting in falsehood. Learn understanding from those generations who provoked Him, blessed be He, until He brought the flood upon them, and until Abraham came and received the reward of all of them. As it concludes in Pirkei Avot, after mentioning “with 10 utterances,” it immediately mentions the matter of these 10 generations to make known how patient He is, blessed be He. Likewise, you should not rely on such vain consolations. Just as these concepts are connected in this verse, so too did the Tanna connect them in Pirkei Avot, for both provide moral instruction to sinners as stated.