Think its almost a given that Bottas won't be in the car next year, the problem that gives Merc this year is that as soon as they tell him that then he stops being the compliant wing man he's been up until now and starts driving for himself instead (and I think we already saw a little bit of that in Barcelona).
Not sure that'll make much difference. Apparently Bottas has admitted to press that during the off season he was considering whether or not he wanted to continue. DC mentioned that he thinks that was the wrong thing to admit in public, as it will affect the way members of the team think as well. It will surely play a part in any decision Merc might still need to make about drivers for next season. It may even be the source of the rumour that was going around suggesting Russell could even be moved in before the end of this season.