Suppose Nokia's chances of survival were 10% without Stephen Elop and 20% with him. That means he is worth 0.1 x value of Nokia if Nokia survives.
If you pay Elop based directly on the performance of Nokia (i.e., he only gets paid if Nokia does well), he'll stay at Microsoft. Why leave MS for an 80% chance of getting paid nothing? Obviously Elop demands some cash up front, a golden parachute, or something of that nature, and a rational board of directors will give it to him.
In 80% of situations like this, the CEO gets paid well for running the company into the ground. But that's a better situation than the alternative, i.e. 90% of similarly situated companies crashing and burning.