There is no such thing as a sure thing in Formula 1. You can only play the percentages. Particularly when it comes to the driver market. Even solid contracts can be broken, if the will is there. Just ask Mercedes and Nico Rosberg.
Basing your career progression on random sequences of events outside of your control is anathema to most in a world where science and logic rule. But even an environment that tries to control events to the nth degree still gets turned upside down by changes of heart, an unexpected triumph of emotion over rationality, or plain old dumb luck.
Valtteri Bottas must have thought he'd missed his shot at the big time when a potential deal to join Ferrari for 2016 fell through. But he stayed with Williams for another year, was set to stay for one more after a brief flirtation with Renault, and now he's suddenly a double grand prix winner with Mercedes, thanks to events set in motion by Rosberg's unexpected retirement last December.