They don’t have to. If you don’t have database replicas that are actively trying to subvert the system, inject bogus transactions, etc. then you don’t have the set of failure domains for which blockchains are in theory useful for.
If you’re running backups for a single organization, you just need replicated data storage on servers owned and operated by that organization. If you’re running backups for a set of users who all trust your organization (e.g. if you’re Dropbox or the like), you also don’t need blockchain.