And Mach 3 vs 2.5. And a 4.4BSD (well, *BSD) user land, although in fairness the original, never released NEXTSTEP 4.0 also had this.

So is the jump between W9X and NT based OSes, both in the kernel, graphics modes (GDI, Direct Draw vs Direct3D to draw the desktop, compositing window managers...) and the like. Specially after Windows 8 where Direct Draw it's slow as hell and you need to use WineD3D which runs ddraw.dll on top of OpenGL. But you can use Win32 on both.

...I'm pretty sure the same would be true in any modern version of NeXTStep had it survived as its own 'brand' (apart from slightly different requirements caused by the hardware the OS needs to run on of course - e.g. running on a handful different Apple devices versus having to work on 'everything').

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.