Talent Knows Talent

The old adage is "As hire As, and Bs hire Cs". If you are unfamiliar with this saying, the quick translation
is that talented people recognize and want to work with other talented people, so they hire them. Top talent realizes that the best way to move their project or company forward is to put the best possible people on it.
And top talent are not afraid of being replaced by talent they bring in. Conversely, Bs (non-top talent) tend to hire people worse than themselves either because they can't recognize talent when they see it, they don't value talent, or they are afraid of it -- or a combination of all these.  My own experience tells me that the adage above is in fact true.

Of course that means the corollary is also true: "If you don't hire As, you are not an A yourself."