You're correct, however:
1. The MinIO image on Docker Hub has more than a billion downloads [^0]. With those download counts, people have almost certainly written scripts that rely on this image existing (including their own Dockerfile! [^1]). Them leaving these images around is just asking for security breaches later down the line.
1b. While, yes, no-one's entitled to freely-available container images, it cost them almost nothing to maintain their existing toolchain for this. Them deciding to pull the plug is purely and entirely a money grab (and a dumb one, if you ask me; look at how the community responded with OpenTofu when Terraform when BUSL).
2. Fortunately, MinIO is a Golang app and can be built with a simple "go install" (though the build instructions in their docs don't align with the build recipe in their Makefile [^2]). However, they could pull a Tesla and make the source that they publish differ from the source that their binaries are built from.
3. They gave NO notice. That's the slimiest part of all of this. Tens of thousands of Kubernetes clusters, and handfuls of enterprise products, run or package MinIO that are now using images that will no longer be updated. All of these people will need to completely change their toolchains to account for that, and soon. That's just not a kind thing to do.
[^0] https://hub.docker.com/r/minio/minio/tags
[^1] https://github.com/minio/minio/blob/master/Dockerfile
[^2] https://github.com/minio/minio/blob/master/Makefile#L179