I don't think Satan wins if the faithful leave. The churches that renounce Christianity will quickly become empty shells and die off over time. But if the faithful remain in fellowship with apostates or heretics when it is clear there will be no repentance, their witness is compromised and their ability to resist Satan's lies is eroded.
Those faithful servants who wanted to remove the "weeds" from the fields, would also have destroyed the "wheat". Could it be that by separating themselves from the so-called "apostates or heretics" in the church, the "faithful" are, in fact, destroying the whole church?
Oh, how simplistic and expected this is. But, sir, the problem is that you are regaling us with a part of Scripture dealing with election, not morality and the nature of faith (you know-fearing, loving and trusting God above all things, including yourself).
Perhaps those words of the Bible of separating yourself from those refusing to repent might be a bit more practical in this matter? I do recall Paul having some pretty harsh words and advice for the congregations to expel those who refuse, in the hope that they repent and stop their sin.