OS9 was really just 8.7 but, as you describe, a stepping stone towards 10, both numerically and with a few nods to the future, e.g. Keychain Access, CarbonLib, and file/printer sharing over TCP.
Carbon was a moving target; each "update" cut more core MacOS tools and added OSX service interfaces. It was quickly obvious that there was no intention of letting MacOS run in the future, so why port to Carbon? many social signals in the developer group at the same time.
https://en.wikipedia.org/wiki/Carbon_(API)