In the case of certificate transparency, the CAs know that the browsers will nuke their business if they’re caught cheating and the design of the system makes tampering evident so they’d primarily only be able to cheat by issuing a certificate without logging it.

I think that’s an important point because it touches on the challenges of a blockchain: if trust can be based entirely on the information in the chain, it can work. If security depends on outside oracles, legal enforcement, or other trust relationships anyway, you don’t need the expense of a blockchain relative to something like a distributed signature system. This is why the supply chain stuff never goes anywhere unless it pivots because the hardest part is trusting your auditors, and the expense / fragility of an always-on networked system isn’t worth it when it can’t solve that problem.