I don't think that hiring or retraining programmers is going to be an issue for most companies. I think that the issue is going to be the pre-existing codebase. Reimplementing old code in Objective C wouldn't involve just rewriting parts of the program, but it would also require testing the new code. In some cases, the investment in optimizing the code would be lost (you can keep algorithms, but it is harder to keep API based optimizations).

But I don't think that Carbon is simply going to disappear. It's more likely to be depreciated first, then pulled out of the SDK, then pulled out of the OS. The process will take many years.

As for the beginning of the end, well, Apple has been losing mainstream developers for decades.