I agree that life goes on, but it's not rare at all. It happens all the time. People leave and things are f-d up for a while.

The thing is, it's not a permanent condition. People adapt, new hires are eventually found to fill the gap, or others who were thought of as not up to the task take the opportunity left by someone's absence to step up in their career.

One person leaving won't sink a company that is large enough. That doesn't mean it won't hurt, however. And how the company handles things after the departure is very critical.

Perhaps more important (and dangerous) is the morale hit that the others will feel as their peers start leaving. There very much can be a "rats-leaving-the-sinking-ship" effect that can be precipitated by a key person leaving.