There's an intermediary scenario to consider: when multiple trusted parties are involved. This is basic computer science, and software engineering: crash and byzantine fault-tolerant systems.
On a related note, blockchains often obscure the true consensus mechanism behind layers of complex jargon. Upon closer inspection, consensus is still built on trusted parties, just not the same ones typically found in the traditional economy. In Bitcoin, for example, you not only have the relatively few powerful miners but also the Bitcoin Core developers, who wield significant influence over which changes are incorporated into the Bitcoin node. For example, enabling the OP_CAT opcode again [1][2].
Parent's point is weak - it's not possible right now for people around the world to hold fiat/stocks in their own centralized, custodial wallets due to stringent KYC/AML, and blockchains currently fill that need for stablecoins, and will fill that need for tokenized stocks, treasuries, etc.
