Redis isn’t used exclusively as a cache. It’s used a lot as a job queue, as an event bus, for session storage, etc. Some even use it as a core source of truth database, though this is rarely a good idea IMO.
I think each of those is nearly as easy to update, except the source of truth. Probably shouldn't ever go on vacation if redis is your source of truth!
Yeah, agreed, though if you’re using Redis as central pub/sub infra in an environment with tonnes of different services who produce and consume events, a migration can get tedious. Certainly not as bad as traditional DB migrations, but doing it under a short timespan could still be a pain.