You're thinking about it wrong. You want the best possible person for the position, and not worry about the
what ifs in life. You can play the
what if game and always get it wrong.
What if this brilliant Phd decides to leave next year? Well, then your company wasn't worth any extended loyalty. But
what if he decides to stay -- are you still going to figure out a way to sabotage his position and make him leave?
Seek only the best person for the position, now. For this now, and not for any possible furture you may invent for the candidate.