I was there when they wanted to do the custom XROS. I remember asking them in a Q&A session exactly why they would build this and I recall the reasoning behind it totally fell flat. Fundamentally it became clear these guys just wanted to write a new OS because they thought it would be cool or fun.
Much of the scenarios they tried to address could have been done with Mach or some realtime kernel or with fuchsia. I recall later on they did consider using fuchsia as the user space for the os for some time.
On another note, there was similarly an equally “just for fun” language effort in the org as well (eg “FNL”). That was also conceived by a bunch of randos who weren’t a bunch of compiler guys that had no product vision and just did it for fun.
Well when the era of efficiency arrived all of this stuff ended.