But that's just it though, they DIDN'T go off and start their own company
That's the opportunity cost to them for working for microsoft. They contributed to the growth of microsoft instead in hopes that it would be a better investment of their time. But Gates is taking their productivity away from them, and attributing it to himself. Then using that as justification for padding his own wallet. If Gates could have done it himself, why didn't he? Why did he bother to hire all those people? It's obvious.