This argument rests on the erroneous premise that "fulness" means every point of doctrine relating to every conceivable gospel topic. Obviously no one book of scripture, or even all the Standard Works together, contain "every conceivable doctrine." In fact, the scriptures are clear that God will yet reveal many great truths (Amos 3:7, John 16:12).
The Book of Mormon gives us a clear understanding of what is needed to truly follow the example of Christ and be "saved" in the Kingdom of God (3 Nephi 11:33). It is required that we have faith in Christ, repent of our sins, be baptized with water and the Holy Ghost and endure in righteousness to the end (see all of 2 Nephi 31 and 32). One might add that the fulness of the gospel, in addition to the above, embraces the atonement of Christ and the universal judgment. (3 Nephi 27:13-2l).
As to the higher ordinances necessary for exaltation (after one enjoys the companionship of the Holy Ghost), it is probably of those ordinances the Lord was speaking when he said, "If thou shalt ask, thou shalt receive revelation upon revelation, knowledge upon knowledge, that thou mayest know the mysteries and peaceable things--that which bringeth joy, that which bringeth life eternal" (D & C 42:61)
As Latter-day Saints we have the promise that a time is coming when this fulness will be preached to all:
For it shall come to pass in that day, that every man shall hear the fulness of the gospel in his own tongue, and in his own language, through those who are ordained unto this power, by the administration of the Comforter, shed forth upon them for the revelation of Jesus Christ (D & C 90:11).The fulness of the gospel that will be taught to all is the "doctrine of Christ." See also 3 Nephi 11:28-40; D & C 10:67.