On the flip side, you can bring in your guys and not have much success. Shanahan brought in a lot of "his guys" including his son. There is plenty of coaching "talent" on the sidelines there, but the on-field results, especially on offense, haven't been there.
Same with the Redskins when Joe Gibbs came back. He found a defensive coordinator who shared his vision in Gregg Williams and allowed him to hire who he wanted to hire for the defensive staff. Gibbs brought in his guys for offense, and two years later, brought in Al Saunders to run the offense.
Don't get me wrong- having a good staff is huge. Sometimes, the right assistants are in place. Othertimes, they aren't. Most coaches who are coordinators first also know other people they've worked with and made connections along the way, so they can find those people. But its not a guarantee to work out all of the time.