If an Nsec gets breached and GrapeRank scores are constantly recalculated the score would go down after a few reports and mutes of the Npub. In parallel the new Nsec could be "upvoted" by follows. It's not perfect, but would allow for a transition to a new key pair. That's how I understood David here: https://fountain.fm/episode/zztOIngx6H7odhl3dFrT?t=3614