I get that it's both unsexy and probably a little uncomfortable, but the thing is, the demand for control that you're expressing here by going against open and collaborative standards really and truly does matter. Like--fundamentally and inescapably, the messaging I'm getting from you is "it's 'open source' but we demand to own it." Why do you need to own the specification to do what you do? What is OpenAPI stopping you from doing? Did you try to work with the OAS folks to improve it to enable whatever (if anything) can't be expressed? Did you evaluate stuff like vendor extensions to it to express those things?
I get that the core of your Standard Library is open-source--but "open source" and "community collaboration" are pretty far apart. I understand the process for OpenAPI's evolution and change over time. What's yours? Whose commits get in? Who decides?
I realize that the likely answers to these questions are trivial in the power-politics/realpolitik sense. Which is why my hackles are up, TBH. Not playing in the sandbox with all the other kids is a clear indication that you have it in you, whether you are or not right this second, to raise barriers to entry against anyone else who'd consider doing something with your tech. And that leads to a fragmentation ("welp, we'll have to fork") that this space well and truly doesn't need. If this was using standard tools and standard approaches--which all totally work with this, by the way, I've written a simpler but spiritually similar thing just by using openapi-js to dynamically generate clients--I'd be cheering you on. As-is? This sure looks like pissing in a community pool for your own benefit.
How am I wrong? (Because, to be clear: I'd love to be. Stuff like this is important.)