This seems to be the answer to the Sforno. Torah sages should be the happiest people in the world. They are constantly growing, constantly accomplishing. They are changing themselves and the world they live in. How can they not be satisfied? How can they feel lacking?
If this group turns to material pleasures, it is indicative that something is missing. It means their focus is off. They have mixed up the “gym” with the “spa,” and they aren’t laboring as intensely in Torah study. Therefore, the Torah isn’t fulfilling them. Because they are too focused on this world, a further downward spiral begins that causes them not to appreciate Torah as much and not to labor as hard in it. They will then feel even more lacking and will need to fill that emptiness with physical pleasures. Eventually, this path will cause them to rebel against Hashem and corrupt the rest of the generation along with them. Moshe Rabbeinu was warning them that their spiritual level and the rest of the nation’s depended upon their keeping their focus, always remembering why Hashem created them.