However, once Google starts to intervene and change things so that the most valued employees change their mind and stay, ... then that changes the equation. Furthermore, once Googlers know they are being "watched" in this way, some may change their behaviour.
A small version of the Heisenberg uncertainty principle? ("observing something changes it").
By the way, Googlers might change their behaviour in two ways. If you know you are leaving, you may try to mask that intention. On the other hand, if you just want a raise, or more challenging work, you might mimic "I am about to quit" signals, hoping to manipulate the system to enhance your situation.