It's not a single private key. There are a lot of validators, even in the genesis block, and we assume that at least 2/3 of these validators are honest.
It still puts an upper bound on the value of the underlying token, as the pressure for these validations to collude increases with the market value of the token in question. An outside attacker compromising keys isn’t the only way for the system to fail.
Would you trust 100 validators with securing $10M? What about $100M or $1bn? The financial incentive to collude keeps increasing as the value increases.
Would you trust 100 validators with securing $10M? What about $100M or $1bn? The financial incentive to collude keeps increasing as the value increases.