No, I didn't think of that yet. But yes I can see the benefit. The thing is that Mock's endpoint configuration format enables you do custom things that afaik OpenAPI wasn't designed for. In Mock's endpoint configuration you can set routes to return different responses based on conditions - say if route such was requested with querystring values such and such, then such response would be returned, and so on.

Imagine for example mocking MediaWiki's API where you have a single "api.php" endpoint, where you determine the "action" of the API operation with querystrings. Mock was used here to mock their API enabling me to easily do E2E tests: https://github.com/dhuan/wikicmd/blob/master/tests/e2e/mock/...