Darwin ditched the old driver stack for IOKit because they thought it was icky to have ObjC in the kernel. That's pretty much entirely up to leadership changes, not technical reasons.

Agreed, the same way Longhorn, Midori and Singularity failed to win the hearts of Windows team, while Android and ChromeOS obliterated their mobile and US school market.

Turns out using a managed userspace is viable, if management is on board to support the development all way through.