>>I've seen this so many times in my career. The A players will stay because they're attached to the things they've created, but the B players will leave and you're left with lots of C players.
The problem is A, B, and C are not at birth positions. People in these buckets rotate across the buckets at various phases in their careers. And the participants in the A buckets are basically people with good leadership- who can train, motivate and get them good products/projects to work on. That given anybody can leave the company and if you have good managers it wouldn't matter.
The real problem happens when you go meta to this. How do you get A level managers. That's a very hard problem to solve and often it also depends of the boss at the very top. The CEO. Many times the revolving door, golden parachute CEOs don't even care. The founder CEO's are tired in most cases. Performing at unicorn level can be brutal on your body and mind on the longer run. And they'd rather semi retire after a few years than continue with that lifestyle. This is where the real problems begin.
In cases where the top most boss gives a damn about the company, the company tends to be A player generation factory. Look at Ford under Henry Ford, Apple under Jobs. Microsoft under Bill G. The boss at the top just needs to get this right else its over.