If any time you were hacked, you fired the previous network security engineer and hired the guy who just broke in, you'd have a network unusuable by legitimate users and still probably vulnerable to skilled attackers.
Who said anything about firing? How does hiring one guy on to join the team == firing of all of the previous guys? Is there a one man network security team rule at Valve that I'm suppose to know about?
If you hire somebody to your security team every time your network is compromised, and never fire anybody, your network security team will outnumber the rest of your company.