Russell was F2 champion and has, so far, beaten every driver he’s been paired with, and consistently, almost embarrassingly so. When he’s tested for Mercedes he’s been very fast and consistent too. He will be a Mercedes Driver next season I’m quite sure.
Who will Russell replace? Bottas is contracted for 2021, I doubt Hamilton will leave when Mercedes are still top of their game, and with the limited changes for 2021 its almost certain the Mercedes will be the best car on the grid again.
I can't see any scenario where Russell will be a Mercedes driver in 2021.