First, Apple announces the "end of usability" far in advance.
They announced the Classic compatibility layer going away early. (And of course people who needed it could continue to use older versions of the OS.)
They announced PowerPC support (Rosetta emulator) going away early. And of course people who needed it could continue to use older versions of the OS.
Lastly, they won't just have *poof* on June 8th Carbon apps will stop working.
They could very well announce it, yes. But there would then be a period of time to port Carbon apps. And Apple has declared Carbon is a dead-end for *MANY* years.
And, finally, of course, if you absolutely *NEED* to use a Carbon app, you continue to use an older version of the OS.
In the previous hardware-based switches (death of native OS 9 booting, Intel switch,) Apple continued to sell previous-system-capable systems for over a year. They continued to sell the OS 9-bootable Power Mac G4 for a year after the line became largely OS X-boot-only. They continued to sell the Power Mac G5 after the Mac Pro came out.