Your argument, of course, is that this one who is erring from the truth is a genuine Christian who will lose his salvation if he does not turn from "the error of his way." Notice how this erring is not specifically spelled out which leads to a vague conclusion. Another argument is that this one who has wandered from the truth is a "professing" Christian, whose faith is not genuine and he will experience eternal death if he is not converted unto salvation. A third argument is that this one who has wandered from the truth is a genuine Christian who needs to be converted from "the error of his way" (not converted to salvation) or else experience premature physical death. This third interpretation supports the argument that the word soul (Gr psyche) may mean "life," and that the clause would be taken as a warning that backsliding may end in physical death. James is addressed the Brethren, but says if anyone "AMONG" you. Among us but not of us?
In regards to turning back the wanderer from error, the arguments for OSAS is the wanderer is either a professing Christian, whose faith is not genuine or a sinning Christian, who needs to be restored. For the former, the death spoken of in vs. 20 is the "second death" (Revelation 21:8); for the latter, it is "physical death" (1 Corinthians 5:1-5; 11:29-32; 1 John 5:16).
In Matthew 26:38, Jesus said: "My soul [psuche] is deeply grieved, to the point of death." Jesus was not saying that His soul was deeply grieved to the point of spiritual death, Rather, Jesus was talking about his human life. Jesus covered our sins in one way by bringing forgiveness for all who believe, yet sins can also be covered in a different way. In Proverbs 10:12, we read: Hatred stirs up strife, But love covers all sins. In 1 Peter 4:8, we read: And above all things have fervent love for one another, for "love will cover a multitude of sins." Where there is strife, there is hatred and unless love prevails, the strife will get worse. Love covers offences and sins when a believer turns back from error.