FatELF exists, though not widely used, and would make more sense than adopting Mach-O, which has worse tooling than ELF. Because of tooling, it'd be preferable

FatELF exists, though not widely used, and would make more sense than adopting Mach-O, which has worse tooling than ELF. Because of tooling, it'd be preferable for Apple to switch from Mach-O to ELF in fact (though they won't do so, because that would require a lot of work and Mach-O is working fine for them).