ṇ is not the substitute of the verb naśa 'to destroy', when ending in ṣ,
When the श् is changed to ष् in नश्, the न of नश् is not changed. Thus प्रनष्टः, परिनष्टः ॥ The नुम् ordained by 7.1.60, is elided by 6.4.24, and श् changed ष् by 8.2.36.
Why do we say षान्तस्य ? Observe प्रणश्यति, परिणश्यति ॥ The word अन्त is taken in order that the prohibition may apply to words like प्रनङ्क्ष्यति, which did end with ष् though the ष has been changed by other rules, and is no longer visible. For here the श् of नश् is changed to ष् by 8.2.36, and then this ष् is changed to क् by 8.2.41; and the नुम् is added by 7.1.60.,