You do make a valid point - however, there are ways to ameliorate (or even eliminate) this risk introduced into the blockchain, by relying on external verification.

One way would be to have each "verified" transaction to be associated with an individual or entity who is providing the "verification", with legally enforceable dire penalties for "submitting" a transaction that is later found to be fraudulent.

So, if a transaction is found to be fraudulent, it can be traced back down to the first transaction in which a fraudulent transaction was submitted as "verified".