As an engineer before Jobs came back — and for a few years too after he was back — there was still a sense that engineers could call the shots with their own frameworks/apps. Copland, Pink, all that — you can dismiss them as failures but they were engineering trying to toss off the "technical debt" of no true multitasking. So, again, it was bottom up and was at least a joy to be an engineer working on the OS at the time.
As I say, for a few years Jobs let engineering call the shots as NeXT became integrated in the OS. I was on the Graphics team then and a whole new graphic architecture and window server were created more or less from scratch (heavily borrowing from NeXT of course since most of the graphics engineering team were NeXT).
At some point though the major changes were in and management began to take over. "Quality" was eventually measured in unit-test code coverage, for example. (Sigh.)