Let's set the record straight: What is Carbon?
Carbon is not a compatibility layer.
Carbon is a re-imagining of the MacOS toolbox. The architects of Carbon had the following technologies to draw from, analyze and improve upon: NeXTSTEP, WIN32 and the Mac OS toolbox.
What they did is systematically take all the functionality from the original MacOS toolbox and wrap it in opaque C89 wrappers. They added const qualifiers. They eliminated the ability access OS structures directly and created accessor functions for the parts that should be private.
They modernized the MacOS toolbox.
And, they took some of the good parts of WIN32 and improved upon them.
In short, they built and delivered a toolbox that enables really big companies like Adobe and Microsoft, who make some of the most popular, useful and widely used software products in the world, to port their software to Mac OS X. If you don't understand that this was one of the most important jobs on the OSX todo list, you're just an ostrich with your head in the sand.
If you eavesdrop civilian computer users, the most common terms in their computer vocabulary are "Microsoft Word", oft shortened to "Word" and "Photoshop". You can add "iPod" to that list, but definitely not Cocoa, or any Cocoa software product. Without Word and Photoshop there is no way that Macintosh marketshare will hit 10%, let alone 30 or 40, or god forbid 60.