I don’t think that’s necessarily always true. I was recently looking into using Redis as a regular database. Unfortunately data is still kept in memory even if it’s persisted on disk, so it was a no-go. You’d be surprised what people want!
I think with Redis, you are starting knowing that your use case needs to be fast. You start with the access patterns and fit that into key/value lookups (or search with modules). You're typically denormalizing the data, duplicating it or aggregating it from other places, so that you can serve operational data fast. Otherwise, just use in-memory SQL.
If what you want is persistent k/v, then there are already tons databases that can do that. Redis has a different use case, so what are you trying to say? That you want to misuse tools?
Are there? What's a good, mainstream, persistent k/v for cheap single-node instances? PostgreSQL's KV support isn't good enough, Cassandra and MongoDB expect to be set up as a cluster and have all the overhead of that, BikeshedDB is alpha and I won't touch their upstream for obvious reasons, CouchDB is pretty much unmaintained, ....
LMDB.
FoundationDB, etcd, consul in single node mode.
I’m always shocked that people use Redis for anything other than an ephemeral cache.