'Then those also who are fallen asleep in Christ have perished.'
So the preceding suggestion that the dead do not rise, that Christ has not risen, and that they are thus yet in their sins, removes any hope for the future. They have died in sin and could only expect to perish. There can be no thought of them being in a state of 'sleep' awaiting the resurrection. Rather they are stone cold dead. They are not 'with Christ' (Philippians 1:23) in an intermediate state of bliss, their bodies sleeping in the grave. Rather they have perished.