Google has literally
thousands of engineers. A significant percentage of those engineers leave every year as part of normal attrition. Google is certainly capable of dealing with the fallout these folks leaving without feeling a thing. While it's true there are some "one-in-a-million" type engineers that work at Google, I don't see any of the well-known ones on this list.
All that said, I commend the folks who signed this letter. If anything, the fact that most of the folks who signed are in relatively junior positions shows how Google has lost its way when it comes to it's morals. The more senior execs feel the pressure of serving Wall Street and have more to lose, while these more junior folks, while taking a risk know they can get hired anywhere else in a heartbeat. It's the kid standing up saying the emperor has no clothes.