JOEL - The church at the time Paul said this was not already "hoplesly apostate" as you say.
Paul states in verse 2 of that chapter:
"And all the brethren which are with me, unto the churches of Galatia:"
Here he specifies that he is talking only to the members of the Church in Galatia. He is wondering why they are allowing themselves to be misled by a gospel different from the one that has been preached to them. They had been taught the true Gospel and Paul was warning them to not accept anything else. Even though Paul was specifically talking to the members of Galatia, Christians of other faiths like to apply his concerns to false doctrines being taught today. This is not a bad idea. But the LDS Church (and angel Moroni) is not preaching "another gospel"; it is preaching the same gospel that Paul taught. But by having latter-day prophets today, we have been able to learn more of it than was recorded in the Bible or more than what Jesus was ready to tell the people of His day.
God dispenses eternal truths "line upon line, precept upon precept" (Isaiah 28) as man is ready to receive it. While Jesus and His apostles were on the earth, their primary goal was to try and teach the simple truths about His Gospel to a simple people who had never heard it or anything like it before. Because of this they were not yet ready to understand or accept every concept associated with Christ's gospel. Paul explained:
"I have fed you with milk, and not with meat: for hitherto ye were not able [to bear it], neither yet now are ye able." (1 Cor. 3:2)
This idea even applied to the Apostles as Jesus explained, "I have yet many things to say unto you, but ye cannot bear them now." (John 16:12) Jesus had something important to tell his apostles, but He wanted to wait until they were ready to "bear" it. However, He died shortly after saying this without telling them what those "many things" were. So when is He going to tell us what those things are?
Paul's statement about the saints in Galatia was an indication that an apostasy was begining in the church. He further spoke of this apostasy that would happen before the second coming of Jesus:
"NOW we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him,
That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;" (2 Thes 2:1-3)
God could not bring about the restoration of His church until this "falling away" had happened and the world was ready to again receive His true gospel. The conditions of the United States in the early 1800s, where people were free to practice their religion, was the prime moment for this to happen. So God restored His gospel to the earth as prophesied by Peter:
"And he shall send Jesus Christ, which before was preached unto you:
Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began." (Acts 3:20:21)
And by John the Revelator:
"And I saw another angel(Moroni) fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people," (Rev. 14:6-7)
And, because we have at least had the Bible during all those 1766 years and knew the basics of the gospel, God was now able to start telling us through His prophets(Amos 3:7), the "many things" that the earlier saints could not bear.