Chazal say very clearly that the evil inclination is the foundation of great and good achievements:
-Abaye said: The greater the man, the greater his evil inclination. [Sukkah 52a]
-Nachman said, in Rav Shmuel's name: "And, behold, it was very good" [Gen 1:31] refers to the evil inclination. Can then the evil inclination be “very good”? That would be extraordinary! Yes, [because] without the evil inclination, no man would build a house, take a wife, beget a family, and engage in work. So said [King] Solomon [in Ecclesiastes]: “And I saw that all labor and all achievement in work was the result of man's envy and rivalry with his neighbor.” (Eccl. 4:4] [Genesis Rabbah 9:7]
-[Why does the Torah say:] You shall love the Lord your God with ALL your heart? [Deut. 6:5]. With ALL your heart means with both your inclinations, the Evil Inclination as well as the Good Inclination. [Berachot 54a]
Yet the rabbis always thunder against the evil inclination and urge us to work hard to defeat it at every turn. Why, given it can be good? You may say, "Because the result would be an absolute stampede! People would say: The rabbi said that it is OK to follow the evil inclination on occasion. “On occasion” means once a week, right? No, I think it means once every five days. No, every three days. Let’s go out and have some real fun tonight!"
I am not convinced. If you say an instinct is "good", then tell people to follow it. But, you will say, people won't know when to stop. The evil inclination is only good in very small dosage, like leaven in the dough. (Analogy in Berachot 17a) Then keep the teaching hidden. Why tell people something is good, then tell them to fight it? Don't tell them anything at all!
So: Why does Chazal tell us to fight the evil inclination tooth and nail, then tell us it is the foundation of all achievement?