Rashi (Beraishis 4:24) writes that the descendants of Kayin are to be decimated as it is indicated in the Pasukim, but we know people that are descendants of Kayin, so how can this be?
You misunderstood the situation. Rashi explains the meaning of the verse to mean that Lemech thought that the Flood would be postponed for many future generations.
If Cain be avenged sevenfold: Cain, who killed intentionally, was given an extension until seven generations. How much more should I, who have killed unintentionally, be given many times seven.
seventy-seven fold: Heb. שִׁבְעִים וְשִׁבְעָה An expression meaning many sevens is used here. So did Rabbi Tanchuma explain it. [This does not appear in extant editions of Tanchuma, but in Yalkut Shim’oni it is quoted from Tanchuma]. In Midrash Genesis Rabbah (23:4): Lemech did not kill anyone, but his wives separated from him after they had fulfilled [the commandment of] propagation, because a decree had been issued to destroy Cain’s seed after seven generations. They said, “Why should we give birth in vain? Tomorrow, the Flood will come and inundate everything!” He answered them, “Have I slain a man for whom I should be wounded? Did I slay Abel, who was a man in stature and a child in years, that my seed should be annihilated for that iniquity? If Cain, who killed, was given an extension of seven generations, I, who did not slay- how much more so should I be given an extension of many sevens!” Now this is an absurd deduction from a minor to a major case, [because] if so, the Holy One, blessed be He, could not exact His debt nor fulfill His word.
In any case Ramban among others says that Noach's wife was descended from Kayin. She was the only direct descendant of Kayin to survive the flood.