A dramatic tension exists between a nation's massive population and a harsh divine decree looming over it. The prophet directs his words either specifically to King Hezekiah [רש״י, רד״ק] or to the entire nation of Israel [שד״ל], delivering a sobering reality. Even if the people are as numerous as the sand of the sea, only a small remnant will ultimately change their ways and return to God [אבן עזרא, רד״ק, מצודת דוד, שטיינזלץ].
Historically, this division plays out between the different kingdoms. The vast majority destined to be lost refers to the exiled Ten Tribes, while the surviving remnant that turns back to God is the Kingdom of Judah, which remained during the reign of Hezekiah [רד״ק, שד״ל]. One perspective even suggests that the immense size of the nation is precisely what prevents the people from repenting. In this view, it is only the tragic loss of the majority that will finally push the remaining few to seek God [שד״ל].
The cause of this massive reduction in population is an absolute and final destruction. There is broad agreement that this devastating event is a firm, decided decree from God, bringing total ruin to the majority.
When understanding how righteousness interacts with this sweeping destruction, two sharply contrasting views emerge. The primary approach among commentators is that the destruction itself will act like a rushing river, striking the people as a matter of strict justice and true judgment. It is a fitting punishment for their past crimes [מצודת דוד, רד״ק, אבן עזרא, שטיינזלץ]. Directed against the kingdoms conquered by the Assyrian king, this ruin operates according to God's exact wisdom rather than random chance, yet it will stop short of washing away Jerusalem [שד״ל].
On the other hand, a completely optimistic approach interprets righteousness not as a feature of the punishment, but as the very tool of rescue. According to this understanding, the merit of the small remnant that returns to the right path will be incredibly powerful. Their own acts of righteousness will surge forward, holding back the final decree of destruction and preventing it from ever reaching them [רש״י, מלבי״ם].