Good Open Source is something like PostgreSQL: It's completely free, you can host it yourself, or you can pay someone to host it, there are multiple competing service providers, most of them contribute to the project, and everyone has access to almost all of the source code. Anyone can start offering a compatible service with minimal investment. If you run into a problem, the source code for everything is public, and you can often fix it yourself.

And then there is something like DenoKV: There's an Open Source version that is designed mostly for prototyping or small scale deployments, and a closed source hosted version designed for production. If you want to use it you have to pay one company and there are no competitors. You are locked in. Theoretically, a competitor could create a compatible product, but the required effort is huge, creating a big barrier to entry. And even if competitors do show up, any new features introduced by the proprietary service will take a long time to trickle down to competing services. If you run into a problem, you have to hope the vendor fixes it.