The argument that I have heard people make is that some ABIs are changing in 64 bit land, such as syscalls or how appkit talks to the window server, so they have to keep some 32 bit frameworks frozen in time in some places, and update them in others. Maintaining 32 bit versions of that is nonzero cost.

That said, I don't agree with such an argument. It seems like seeking out technical excuses for crappy user outcomes. In another thread on this topic I said: in any job there are parts that are fun and others that are not so fun but necessary. They should try to keep customer apps working.