If an attacker buy lots BKS, he/she has less incentive to attack. Just like how PPC this kind of PoS deals with 51% attack. There is a concept “long con”, : a man does well for 10 years and suddenly become evil. Reputation is important but not sufficient.
I agree that BKS collateral is more theoretically sound than no collateral. Also, with a BKS collateral there would be a minimum amount you can deposit without fear = the sum of the required signers collateral. The assumption being that if the signers act without the blockchain telling them to then their collateral is destroyed.
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Reputation + Collateral, plus the future profit/income as a signer.