You're right - you can model any data as append-only though. Granted, in many cases it will require you to seriously sit down and remodel your data. Nobody's claiming it's going to be SQL and ACID transactions :) It's more likely going to be collaborative append-only logs based on CRDTs.

There are examples of the use cases you mention being built with decentralized technologies. [1][2]

[1] The various cryptocurrency wallets and exchanges

[2] https://openbazaar.org