The ‘basic Unix/C stuff’ includes most of the architecture-specific code, in both the kernel and userland, so keeping it around would have a large cost. And th

The ‘basic Unix/C stuff’ includes most of the architecture-specific code, in both the kernel and userland, so keeping it around would have a large cost. And there wouldn’t be much point: command line tools are probably much more likely to be 64-bit than GUI apps, as they’re more likely to be open source (allowing recompiling even if the original developer is gone), and more likely to be portable between operating systems (reducing the chance of running into either architecture-specific dependencies, or macOS APIs that happened to be deprecated with the move to 64-bit).