In order to really leverage a nonfree (proprietary) or more-free (AGPL/SSPL) license you have to have a substantial thing to protect. If you try to protect something trivial, your competition will just implement it themselves, unless your price is low enough to make that not worth it. Redis is relatively trivial, it is a REmote DIctionary Service. Amazon could have written their own Redis quite easily.

They didn't, because the idea is sufficiently non-obvious, but ideas are protected by patents, not copyright.

Even RMS recommended using LGPL in some cases to maximize overall freedom by not making your competitors copy it. In the case of Redis, GPL probably would've maximised freedom (but not revenue) as Amazon still could've used it and released any changes they made.

Valkey has diverged from Redis, gaining features like vector search and multithreading.