[x0x0]

I think it's purely a simplicity thing.

Right now, most rails setups with decent traffic will have frontend boxes, a sql db, a KV store (redis or memcached), and a cache store pointed at the kv store, with, annoyingly, very different usage patterns than typical KV store usage, eg for maintaining api quotas or rate limiting.

Disk performance has gotten fast enough and SQL performance has gotten good enough that there's a movement to drop the KV store and split the usages (for traditional KV use and also backing a cache) to the sql db and disk, respectively. Plus new nvme disks are almost as fast and still much cheaper than ram so you can cache more.