If you don't want things like client/server generators and documentation, then sure it's not great

No, it's not. A description of a single method can often span a couple of screens, and still not cover everything.

In addition, YAML is not easily composable, so you end up with files that are megabytes in size. This is completely useless for humans, unless you start using third-party tools to split the file into parts.

Protobuf-based protocols are also much better specified, and they don't have multiple ways to pass in data. Meanwhile, OpenAPI supports: headers, path queries, multiparts, forms with various encodings, uploads, etc.