The apostasy of the Nephites is particularly disastrous because of the great blessings that they had received previously. This is the enduring difference between the majority of the Lamanites and the Nephites. The Lamanites who had not known the gospel never did willfully rebel, and therefore do not come under the condemnation of the Nephites. For Mormon, this greater knowledge is the reason that this rebellion will lead to their destruction, but the Lamanites would be preserved.
In this historical conception, Mormon presents an earthly parallel to heavenly salvation and destruction. Note how Abinadi describes the relative eternal fates of those who have received the gospel as contrasted to those who have not:
Mosiah 15:26-27
26 But behold, and fear, and tremble before God, for ye ought to tremble; for the Lord redeemeth none such that rebel against him and die in their sins; yea, even all those that have perished in their sins ever since the world began, that have wilfully rebelled against God, that have known the commandments of God, and would not keep them; these are they that have no part in the first resurrection.
27 Therefore ought ye not to tremble? For salvation cometh to none such; for the Lord hath redeemed none such; yea, neither can the Lord redeem such; for he cannot deny himself; for he cannot deny justice when it has its claim.
Whether Mormon’s application of the precise phrase “willfully rebelled” is intentional or coincidental, the conceptual parallel is clear. The fate of willful rebellion is destruction. Now that the Nephites are in such a state, they are ripe for destruction, and Mormon is fully aware that in historical terms their destruction is soon at hand.