“Nor sinners in the assembly of the righteous” (Psalm 1:4)
Presently many unsaved sinners do than in the assembly of the righteous and sometime lead. “But when the wheat sprouted and bore grain, then the tares became evident also.” (Matthew 13:26) But that is not how it will end: “So just as the tares are gathered up and burned with fire, so shall it be at the end of the age.” (Matthew 13:40)
The phrase being considered is a parallel thought to “wicked will not stand in the judgment.” (v.4) Because they have no hope in the judgment, then there is no hope of them being among the righteous. The consequence for them will be dire, eternal separation from God. The consequence for those of us who believe is that they will not be among us any longer inciting us to evil, deceiving us with falsehood, or leading us astray.