In the case of steem, trust broke when the exchanges staked their steem and voted in multiple copies of Justin sun to fill the top 20. It is possible trust could have been recovered after that point, but the continued use of Sybil witnesses made it impossible
Maybe it's naive, but an actor posing as multiple witnesses is a lie. Lying breaks trust. Using the Steemit, inc stake for voting would break trust with a number of steem community members, but may not have been enough for layer zero to engage protective action
The greatest thing about DPoS is that it puts the human element on chain. It doesn't have to stay as rigid as other systems. That means we can forgive, forget, and negotiate through a number of scenarios. As such, only the DPoS community can really decide when trust has been broken for them
I don't think there can be a universal measure of when trust is broken, but for cheeses sake, can we all at least agree that a successful 51% attack used to initiate a Sybil attack is definitely past the threshold? 😄 The answer is "no". We cannot all agree on that. Exchanges see Steem as a legitimate deployment, and there are a number of users happy with the benevolent dictator