Lewis is in for 2 years, I believe. Right now, Bottas is doing just what they need him to do, be a consistent #2 and cause no huge waves. Bring Russell along too soon and you very well could end up with a RB situation like Ver/Ric (or hell, Ham/Ros). Lewis is going to fall off pace at some point, but not quite at the moment. But if Mercedes doesn't do something, they run the risk of losing him.

The wild card is the new car and regs. Nobody knows how they will run and who will get a handle on them. Make too many changes to the team at once and it can all go tits up pretty quickly