Wonderful post and I will be taking inspiration from it. Surprised not to see TypeSpec https://typespec.io/ mentioned, which is a TypeScript-like schema language that I like to describe as "what if OpenAPI was good". I'm guessing they considered it and decided building their own would be both simpler and more flexible. The cost of BYO has come down a lot thanks to agents.
Which parts of Openapi does it fix?
Love TypeSpec, agree it makes writing OpenAPI really easy.
But I’ve moved to using https://aep.dev style APIs as much as possible (sometimes written with TypeSpec), because the consistency allows you to use prebaked aepcli or very easily write your own since everything behaves like know “resources” with a consistent pattern.
Also Terraform works out of the box, with no needing to write a provider.