The reference to this wicked generation locates the saying historically. I would suggest that Jesus is talking about the generation of Jews that would suffer the horrors of the war against Rome. He casts out demons from Israel, but he warns that these homeless spirits will return with a vengeance and wreak havoc. The derangement of the possessed who confronted him was nothing compared to the self-destructive madness of those who, a generation later, rebelled against Rome. Josephus describes the lead up to war in these terms:

Now when these were quieted, it happened, as it does in a diseased body, that another part was subject to an inflammation; for a company of deceivers and robbers got together, and persuaded the Jews to revolt, and exhorted them to assert their liberty, inflicting death on those that continued in obedience to the Roman government, and saying, that such as willingly chose slavery ought to be forced from such their desired inclinations; for they parted themselves into different bodies, and lay in wait up and down the country, and plundered the houses of the great men, and slew the men themselves, and set the villages on fire; and this till all Judea was filled with the effects of their madness. And thus the flame was every day more and more blown up, till it came to a direct war. (War, 2.13.6, emphasis added)