I don't think that this is a case where you want to trade off security for usability, and I don't think that generally those cases exist. You always want more usability. The problems that OPM has are massive, their solutions need to scale. Solutions like what you propose make their solutions stop scaling.
Security people (and armchair security people especially) think that security is the end goal. Security is never the end goal. The mission is the end goal. Security is only useful in so much as it helps you achieve the mission. Sometimes security needs to get out of the way when the mission needs to get done. It is always a tradeoff. The personnel clearance system is already such a tradeoff, a background investigation system that needs to scale to millions of people. Good luck making it "ideally secure."
There were data hygiene and application/network security best practices that OPM should have followed. In hindsight, they would have been way cheaper than the response to a breach like this. Responding by taking all the systems offline would most likely be far more expensive to the system as a whole than a future breach.
Security people (and armchair security people especially) think that security is the end goal. Security is never the end goal. The mission is the end goal. Security is only useful in so much as it helps you achieve the mission. Sometimes security needs to get out of the way when the mission needs to get done. It is always a tradeoff. The personnel clearance system is already such a tradeoff, a background investigation system that needs to scale to millions of people. Good luck making it "ideally secure."
There were data hygiene and application/network security best practices that OPM should have followed. In hindsight, they would have been way cheaper than the response to a breach like this. Responding by taking all the systems offline would most likely be far more expensive to the system as a whole than a future breach.