The situation with CodeWarrior wasn't one where Apple was "dependent" on another company. CodeWarrior was simply a better compiler and development environment than Apple's own tools, so it was more popular with developers. Apple supplied the Mac development SDKs for use with CodeWarrior.
A bigger issue was for application developers who used CodeWarrior when the product was discontinued - they had to port their application to build with a different set of code tools and learn how to use a different IDE. (This was on top of issues around Carbon vs Cocoa.)
(Sorry, lost track of what I snipped above.)
This latter point was exactly the problem with CodeWarrior. It offered an alternate framework that interfered with adoption of native Cocoa APIs. It was a layer between developers and the OS that defined the platform rather than Apple being free to define it. (Sound familiar?) Once Apple made the move to OS X, CodeWarrior had to die so Mac OS X could prosper. Developers had to be dragged kicking and screaming into the future.