The memory that XNU and Darwin are technically open-source projects is a curse that brings one only suffering.

Does not accepting contributions make you not open source? I would assume if the license allows you to fork it, that makes it open source (as opposed to "shared source" licenses that basically say "look don't touch")

I feel like Apple wouldn't want to make full Linux work on their hardware, but they could enable their Darwin kernel to emulate Linux syscalls and provide a way to boot into a mode that basically loads the kernel and whatever Linux shell you want

This path is already taken and it didn't sell Apple hardware in masses.

https://en.wikipedia.org/wiki/MkLinux

MkLinux was first released in 1996, and discontinued in 2002.

I would argue that things have changed significantly since then.