You made that jump by insisting that only true believers persevere. The saved Galatians must not have been true disciples (though Paul says they were) because they did not "continue in My word".
The apostle Peter denied Christ 3 times and all the remaining 11 disciples (after Judas betrayed Jesus) were said to "fall away" (Matthew 26:31-35) when Jesus was arrested. According to your logic, they did not continue and lost their salvation, but that was not the case. Later in Galatians 2:11-15, we read that Paul had to rebuke Peter because he was not being straightforward about the truth of the gospel. When certain men came from James, he would eat with the Gentiles; but when they came, he withdrew and separated himself, fearing those who were of the circumcision. 13 And the rest of the Jews also played the hypocrite with him, so that even Barnabas was carried away with their hypocrisy. According to your logic, where did that leave Peter? You need to stop jumping to biased conclusions.