John Carmack's arguments against building a custom XR OS at Meta

What would be the real advantage of a custom OS over a Linux distribution?

The OS does process scheduling, program management, etc. Ok, you don’t want a VR headset to run certain things slowly or crash. But some Linux distributions are battle-tested and stable, and fast, so can’t you write ordinary programs that are fast and reliable (e.g. the camera movement and passthrough use RTLinux and have a failsafe that has been formally verified or extensively tested) and that’s enough?

Maybe not applicable for the XR platform here, but you could add introspection capabilities not present in Linux, a la Genera letting the developer hotpatch driver-level code, or get all processes running on a shared address space which lets processes pass pointers around instead of the Unix model of serializing/deserializing data for communication (http://metamodular.com/Common-Lisp/lispos.html)