Actually, the Macho file format was multiarch by design (On Windows we're still stuck with Program Files (x86))..

Anyway, before dropping 32bit, they've dropped PowerPC.

Another consideration, Apple is the king of dylib, you're usually dynamically linking to the OS frameworks/libs. so they can actually plan their glue smarter so the frameworks would still work in native arch. (that was really important with PPC->Intel where you also had big endian...)