Right from the off, Bottas has been Mercedes biggest (possibly only) target.
1. He is experienced and reliable
2. He is quick (given decent equipment)
3. He is laid back and cool, so he can cope with the BS from Hamilton
4. Williams is a merc team, so prising him away will be quite a simple task as Williams want to keep Merc happy
5. Massa will come back to keep and experienced driver alongside the rookie. Massa only retired because at the time Williams had signed Stroll and Bottas already had a contract. There was no (or very little) bad blood between Massa and the team