Well, I gather Ocon is under a two-year contract with Renault so he is out of the picture for the moment (while Valtteri is under yet another one-year contract). So I think by default, Bottas is there for the next two years. I am not sure who they would choose beside him for 2021 and I am assuming the Hamilton is staying there until retirement.
Next season when everyone's contract is up even Bottas' seat is not guaranteed. Alonso says he will consider returning and Mercedes have shown they are not afraid to bring a driver out of retirement as part of their marketing.
Merc want drivers who would represent the brand in the right way but also the best drivers available. If Hamilton were to go then they would want a world champion driver that can still cut it at the top - that excludes Kimi.
Ocon might take Bottas' seat as Renault said they want to promote an academy driver into the team in 2021 although naturally that would mean Riccardo goes to make way who also might look at Bottas' seat
For Bottas to keep his seat next season he has to show to he can sustain a championship challenge and not have too many off races against Hamilton