Wasn't Rosberg already at Mercedes when Hamilton joined?
Bottas's performance has declined over the last 2 years. Previous years he was pushing Hamilton. This year and last he's been no where near.
yes upto the point Lewis joined the team Rosberg although well respected wasn't really considered championship material, he was there for consistency, Schumacher was already passed his best when he came out of retirement and drove alongside Rosberg.
That all changed when hamilton joined, Hamilton was supposed to come in and wipe the floor with Rosberg, which didnt happen, everyone was surprised how far Rosberg pushed lewis, it also made lewis stand up and take notice. This forced lewis to start mind games off track as he has done with previous team mates who have been a bit close for comfort (he tried it with Button, but Button shrugged him off). This is where Rosberg struggled and the reason why, when rosberg finally scored his championship he waved good bye, it wasnt good for his health he'd proved his point and didnt feel the need to do it any more, especially as he had a family by then.