The Ramchal writes elsewhere that Gehinom is a just a temporary cleansing process for the vast majority of souls, so it would seem to be inaccurate to describe it as "destruction". I believe that he also writes elsewhere (and also touches on it in the previous chapter) that a person's deeds in this world determine the wholeness of his soul and spiritual level in the next world, and that while the spiritual damage of misdeeds can be corrected through the cleansing of Gehinom, a lack of positive action/spiritual progress leads to a permanent lacking, for one can only actively improve his soul in this world. For example, if someone is tempted to transgress but he resists, it's not only that he avoids staining his soul; he actually elevates his soul. If he fails the test and transgresses, he has lost his opportunity to attain the higher level that was made available to him.
I would suggest that in this sense, lack of Zehirut leads to destruction: he hasn't become what he could have, his soul will always be 'missing' the part/level that could have been achieved, even after being cleansed in Gehinom.