Well, you mention double-spend problem, that is a very narrow scope of applicability for a centralized database. There is a huge public, cryptographically secu

Well, you mention double-spend problem, that is a very narrow scope of applicability for a centralized database. There is a huge public, cryptographically secure, append-only, verifiable centralized database (in fact, they are several), used by Certificate Transparency. Spoiler alert: Its a sparse merkle tree in MySQL.