Media recently asked Kimi on his intention for next year. He said he had decided but would not let the cat out of the bag yet.
IMO it's too early in the year for Ferrari to have agreed to extend him for another year, so I think his decision is to pull the pin.
Vettel is clearly Ferrari's future and Kimi wont enjoy playing second fiddle and he has always said he will stop when he no longer enjoys racing.
Vettel recently indicated he was happy at Ferrari and could see no reason for team change.
Both Vettel's and Raikkonen's contracts are coming up at end of this year as is Bottas'.