Well for starters, it would have better memory management. The XNU kernel's memory manager has poor time complexity. If I create a bunch of sparse memory maps using mmap() then XNU starts to croak once I have 10,000+ of them.
Please re read the comment you’re responding to about how the kernel would have diverged significantly even if they did use the Linux kernel. Unless you think a three decade old kernel would have the same characteristics as today.
What benefit would it have had at the time? What guarantees would it have given at the time that would have persisted three decades later?
