I think their rationale is that they don't want to maintain 32-bit and 64-bit code forever.

But IMO, Apple should have removed Carbon last year (six years after deprecation). That would have made the removal of 32-bit Cocoa a bit less traumatic.

Even though Linux distributions have been slowly dropping support for running on 32-bit systems nobody is talking about dropping 32-bit multilib support. The cost is primarily disk space, and Apple could just make 32-bit libraries an optional download.

Making CarbonLib an optional component several years ago would have made it clearer to users that "this is going away"

“Users” don’t know what CarbonLib is. When Apple announced that they weren’t porting CarbonLib, developers knew.

Users would have noticed when Application X stopped working unless they downloaded this thing that the download page for makes very clear will be unsupported in 2 years.

Do you really think any app developer would ask the users to download a dll from the web to make their app work? They would jusr bundle it like installers bundle the Java runtime or Windows apps bundle the compatible version of msvcrt32.dll